On Saturday night, July 21 2001, Richard Moth and his girlfriend Nicola Doherty left their friends in a Genoa bar because she was feeling tired and wanted an early night. The two London care workers were among vast numbers of people demonstrating at that year's G8 summit. Protesters against corporate globalisation had been following the leaders of the world's major industrial nations to their annual meetings in different parts of the globe since 1998. But this was a gathering on a scale unlike anything that had preceded it. The organisers put the numbers at 200,000; the police said there were 100,000. And it had turned ugly. Even though - or perhaps because - there were 20,000 police in Genoa, including reinforcements drawn from all over Italy, the summit protests became a bloody battleground.

Demonstrators blamed heavy-handed, provocative policing. The authorities pointed the finger at a minority of violent protesters who had, over the course of the summit, looted shops, vandalised the streets and pelted police with rocks. The authorities also didn't much like the repeated assaults made by protesters on a high steel fence, topped with barbed wire, that had been erected to isolate the G8 leaders. Tensions were running high. On Friday, the first day of the meeting, a demonstrator, Carlo Giuliani, was shot dead by a policeman, and more than 200 others were injured - around two-thirds of them protesters.

Doherty and Moth were staying at the Armando Diaz school complex, which was being used as a press centre and makeshift dormitory by the Genoa Social Forum, the umbrella group behind the demonstrations. Doherty, then aged 27, and Moth, 32, had been at the main demonstration that day. It had been mostly peaceful, but there had been clashes between police and protesters, some of them fierce. The two Britons settled down at the school, unaware that it had been chosen for an organised police attack: at least 150 police officers assaulted defenceless and - as was later proven - peaceful protesters. Some of the most senior officers in Italy witnessed police going into the building, and the raid was so ferocious that, as photographs taken afterwards show, it left walls and radiators in the school spattered with the blood of the victims.

At around 11.30, Doherty says, she and Moth were still awake and had just zipped their sleeping bags together. "We heard a lot of noise," she says. "We looked out of the window and saw that the whole street was covered in police in riot gear." Everyone in the building immediately got down on the floor. Moth climbed on top of Doherty, to protect her. "The police came in and started beating people with batons and kicking everyone, and people were crying and begging them to stop, but they just carried on," she says.

Doherty was hit on her arms, legs, side and hips, hit so hard on the arm shielding her face that her wrist was broken. Moth was to need stitches to his head and leg, while his back was beaten black and blue.

"It was relentless," he says. "Blow after blow after blow. When the first lot [of police] were worn out, some others came along and had their turn as well." It was apparently more than some of the police could stomach. A court has since heard from an eyewitness that the deputy commander of an antiriot unit sent up from Rome tore off his helmet, shouting "Basta! Basta!" ("That's enough! That's enough!").

Moth and Doherty were not even among the most seriously injured. Mark Covell, a 37-year-old volunteer with the Indymedia news network, was set on by police outside the building. "I was attacked three times in 15 minutes. First, about eight officers beat me to the ground. There was a pause. I just lay there. Then around 20 officers ran past and one of them came back. He hooked his boot under my ribcage and tossed me into the air, then the others joined in and I was used as a football. Eight of my ribs were broken and I got a punctured lung. The last attack, though, was the most serious. One cop hit me with his baton on the back of the head and another kicked me in the jaw. Ten of my teeth were knocked out or broken. There was blood in my throat and I couldn't breathe," he says. "I thought that was it, that I was going to die that night."

He lost consciousness and did not come round for 14 hours. Four years later, he is still far from recovered. "I have trapped nerves and tendons in my spine, and a hand that is going to need to be re-broken," Covell says. "They say my post-traumatic stress levels are still sky-high and that I'm going to be in therapy for years. For me, Genoa is still going on."

Of the 93 people arrested that night at the Diaz, 62 needed medical treatment. Of those, 28 were taken to hospital and three, including Covell, were put on the critical list.

The raid happened in the heart of Europe, in a democratic nation with the eyes of the world upon it. Yet, fortunately for Italy and Silvio Berlusconi's government, the whole affair was to be swiftly forgotten. Less than three months later, the World Trade Centre was attacked and after that every other event began to take on reshaped proportions.

Back in Italy, though, the sequence of events that began at the Diaz school has not been forgotten. It has given rise to two sensational court cases in which more than 60 officers, including some of Italy's top policemen, stand accused of violating the very principles of law and order they are sworn to uphold. The second of two preliminary hearings is due to open next Thursday and deals with what happened after the Diaz school raid, for the nightmare did not stop there.

Forty of the protesters were taken to a holding centre at Bolzaneto, six miles from Genoa, where they were put with others arrested elsewhere in the city. Here, according to prosecutors, they were further humiliated and abused, physically and verbally.

Moth, who had been taken out of the Diaz school on a stretcher, says he "had to be held down, screaming with pain" at the hospital while they stitched him up. Then he was taken to Bolzaneto in a police car. There, despite his injuries, "I was made to stand for hours spread-eagled against a wall." On his way to and from the lavatories, he says, he ran the gauntlet of prison officers who kicked out at him as he passed. Other detainees have variously testified that they were spat at, warned they would be raped, and threatened with anal and vaginal penetration with truncheons. The most senior doctor present is being charged with insulting detainees during their medical inspections and failing to inform the authorities after protesters were sprayed with asphyxiating gas in their cells.

At Bolzaneto, a new and sinister aspect of the whole affair emerged. The prosecution claims that some of the detainees were forced to shout "Viva il Duce" (Long live the Leader) and other slogans in support of Italy's fascist dictator Benito Mussolini. They were also allegedly made to sing a song. This is how the prosecutors say it went: "Un, due, tre. Viva Pinochet. Quattro, cinque, sei. A morte gli ebrei." ("One, two, three. Long live Pinochet. Four, five, six. Death to the Jews.")

Extreme rightwing attitudes crop up in many parts of the Italian police and security forces, but are particularly prevalent in the antiriot units that were deployed at Bolzaneto. In Italy, those with authoritarian ideas about "law and order" often align with Gianfranco Fini's National Alliance party, the direct successor of the fascist movement of the 1930s.

Crucially, the foreigners at Bolzaneto were told to sign a document waiving their rights. "It wasn't translated," says a British lawyer representing two of the victims. "It was a special form for the G8 summit, so it had been prepared in advance." Among the questions and pre-filled answers on the form, in Italian, were: "Do you want your embassy and consulate contacted?" "No." "Do you want somebody informed of your detention?" "Nobody." "Do you need an interpreter?" "No, because I understand Italian very well."

Forty-seven people are facing indictment in connection with what happened at Bolzaneto. They include police officers, prison guards and medical staff. Unsurprisingly, some of the victims have described what happened to them as torture. But, in doing so, they have created a difficulty for the prosecutors. One of several gaps in Italy's laws and regulations exposed by events surrounding the G8 protests is the lack of a statutory ban on torture. So, as torture is not a crime in Italy, if the staff at Bolzaneto are shown to have been torturers, it would not be grounds for prosecution.

Proceedings against those allegedly involved in the Diaz school raid are more advanced: 28 senior officers are variously accused not just of trespass, false arrest and inflicting or authorising grievous bodily harm, but of trying to justify what they'd done with a pack of lies. The preliminary hearing began in June last year and continued until September; after listening to an outline of the evidence against the defendants, a judge in Genoa ordered that all 28 be sent for a full trial. It is due to start in April this year. All the defendants in both trials deny the charges.

The sensitivity of these court cases is hard to overstate. The charges raise questions of the gravest kind about both the Italian police and the Berlusconi government. In British terms, it is as if the head of the prison guard, high-ranking officers from Special Branch and Scotland Yard, and assistant chief constables from several regional forces were all in the dock together. The defendants in the Diaz trial include Francesco Gratteri, number two of the anti-terrorist division of the Italian police, several squad commanders from police headquarters in Rome and past or present assistant commissioners from Bologna and Genoa itself. The Bolzaneto hearing will have before it General Oronzo Doria, the commissioner of the penitentiary police, and an assistant commissioner from Turin.

Never has there been a calling to account of the Italian police on this scale. In the past, individual policemen and women, or small groups of officers, have been put in the dock, accused of falsifying evidence or mistreating suspects. But no one, until now, has ever thought to arraign such a broad cross-section of the top command. In many countries, it would be inconceivable. But Italy, for all its chaos and corruption, has some remarkable institutions, and one of them is a wholly independent prosecution service. Unlike in Britain, a prosecutor who suspects an offence has been committed does not have to seek permission from anyone - least of all the government - before opening an inquiry or, indeed, bringing charges.

But to bring proceedings this far, the prosecutors of Genoa say that they first had to demolish a wall of lies and obfuscation erected by the defendants.

Dan McQuillan, 40, from north London, who suffered severe injuries to his head and body, describes the protesters in the Diaz school that night as "a right bunch of do-gooding people: care workers, social workers; young, hopeful people trying to do something for society in their job, and who had the social conscience to go on demos". According to the police, however, the Diaz school was an operational base of the "Black Bloc", a loose anarchist grouping that often uses property damage directed at corporations as a protest tactic. This bloc was held responsible for much of the destruction in Genoa. That destruction, and the violence against police that accompanied it, is the focus of separate proceedings in which 25 demonstrators are being tried.

There is some evidence that the rank-and-file officers who were ordered into the school thought they were being given a chance to get even with the most aggressive of the protesters. Covell says that, just before he was set upon, the officer who grabbed him said, in English: "You are Black Bloc. We are going to kill Black Bloc."

A police charge sheet stated that the occupants of the Diaz school had resisted arrest and were in possession of various "arms, offensive weapons and other material, illegally held within the school". A list included Swiss army knives, hammers, shovels, nails and other carpenter's tools (the school was being renovated over the summer holidays), an assortment of black clothing (the uniform of the Black Bloc) and, most damningly, "two bottles (Molotov cocktails) in the entrance hall on the ground floor". As if that were not proof enough of the threat posed by the alleged subversives holed up in the school, an officer from Rome, Massimo Nucera, came forward to say that he had been attacked by a demonstrator with a knife. He produced a slashed jacket to prove it.

At a press conference the following day, police said all 93 protesters arrested at the school could be charged with conspiracy to bomb, and jailed for five years. Five days after the demonstrators' ordeal began, they were released and, in the case of the foreigners, briskly deported.

But as prosecutors and judges scrutinised the police claims, they concluded that they were baseless. Italian justice is slow. It took almost two years for the affair to reach court, but in May 2003 a Genoa judge ruled that not one of the 93 people arrested had been involved in violence. Judge Anna Ivaldi found, "Nothing has come to light against them that would lead one to suppose the existence of associative relations with those who sacked and wrecked the city."

In a speech towards the end of the preliminary Diaz hearing in September 2004, the prosecutor, Enrico Zucca, noted sardonically that no single operation "in the long history of the Italian police" had ever resulted in so many wrongful arrests. Returning to the attack at the court's next session, Zucca argued that the police had conspired to justify the brutality of the raid with trumped-up evidence. As he and the detectives working for him had dug into the affair, discovering contradiction after contradiction, they had found "a disturbing yet simple answer: the police officers must have lied".

For a start, said Zucca, there never were any Molotov cocktails at the school - or rather, none until the police planted them there. The police had reported finding four petrol bombs in Genoa on July 21, two in the city centre during the day and two at the school in the evening. But they could produce only two. An explanation was eventually provided by Pasquale Guaglione, an assistant police commissioner from Naples who, like many other senior officers, had been drafted in from outside to help police the demonstrations. He told prosecutors the bottles taken away from the school were the same as those his unit had picked up earlier in the day at the scene of fierce clashes between demonstrators and police. They were wine bottles and he recognised the labels - Merlot and Colli Piacentini. Then a junior officer is said to have admitted during interrogation that he brought the Molotov cocktails to the Diaz school on the orders of a superior. Finally, an assistant commissioner from Rome, Massimiliano Di Bernardini, said that he had seen the two bottles being carried by another senior officer, Pietro Troiani, at the school on the night of the raid. This made Di Bernardini a key prosecution witness. Five days before the preliminary hearing began in June, the assistant commissioner was involved in a motorcycle accident that has left him in a coma ever since.

The Molotovs are not the only allegedly falsified evidence. The prosecution claims that the knife attack was faked, too. It has produced forensic findings which, it says, show it was the police, and not a demonstrator, who slashed the jacket to provide evidence of an assault.

Lawyers representing the victims add that very basic details given by police officers of events that night showed contradictions. There were claims that, prior to their entering the building, they were attacked by stone-throwing protesters inside. But, says Massimo Pastore, Covell's lawyer, "Some [police officers] said the protesters threw a lot, some said a few, and others said they didn't notice any stones." There is video evidence of the police entering the building in which no stone-hurling is visible.

According to one of the defence lawyers, Silvio Romanelli, "there were at least 150 police officers inside the school". Yet more than three years after their ferocious attack, nobody can say for sure who - or how many - they were. Police handed prosecutors a list of men involved in the operation. That it was a partial list was established, says Pastore, only because "one policeman would say, 'I was with X', then you look at the list and see that X was not on it". Prosecutors also asked for photographs of the men on the list. "They were for the most part pictures that not even their mothers would recognise," Pastore says. In one case, prosecutors obtained a video of a long-haired, plain-clothed officer, caught in the act of hitting a protester. But despite his distinctive appearance, and despite there being only a small number of officers permitted to work in plain clothes, no police unit could identify him. The prosecutors have accused the police of building "a wall of omerta [the term for the mafia vow of silence]" around the case.

By far the biggest single obstacle to the identification of those who carried out the beatings is that they were all masked and none of them carried a name or number on their uniforms. Italian law does not require them to do so. Out of the 28 officers on trial for the Diaz school raid, just one is accused of inflicting bodily harm.

It can be argued that this is as it should be; that the rank-and-file officers were carrying out orders from their superiors. But it invites the question: were those superiors responsible for sanctioning the raid, or were they in turn carrying out orders given to them by someone higher up? Pastore says: "It is hard to think that any police, anywhere in the world, can do a thing like this without political sanction."

Genoa was the high-water mark of a swelling anticapitalist, or social justice, movement that had inspired mass action around the world. Similar protests had already shut down a WTO meeting in Seattle in 1999 and forced the early closure of an IMF meeting in Prague in 2000. Did someone, somewhere, decide that enough was enough; that extreme measures needed to be taken?

Even before the raid on the Diaz school, Guardian correspondents in Genoa for the summit had reported "highly aggressive policing tactics, including water cannon, tear gas and clubs". Richard Parry, a lawyer representing some of the Britons, argues that "when they start attacking a peaceful demonstration with tanks and helicopters, when they start dropping tear gas on to pensioners, you realise this is a tactic to show that the state, by which I mean the international world order, is not going to accept any further interference with its plans."

Silvio Berlusconi's newly elected and assertively rightwing government was certainly in a mood to flex its muscles. But did it actually plan and authorise the raid on the Diaz school? Many of the victims believe it did, and they point to media reports at the time indicating that Berlusconi's deputy, the former neofascist Gianfranco Fini, was in the police operation centre."We know that [Fini] was there, but whether it was for a few minutes or longer, we don't know," says Senator Francesco Martone, a member of the Green party, in whose constituency the school falls. The Greens and other opposition politicians wanted a parliamentary commission of inquiry to establish what political responsibility, if any, there was for the events of that night. Berlusconi's allies and followers used their outright majority to block it. Since then, they have made it quite clear what they think. Ignoring demands from opposition MPs that the police officers on trial should be suspended until legal proceedings are over, the Berlusconi government has allowed several to be promoted to key positions. Francesco Gratteri was made deputy head of the antiterrorist division after the Diaz raid. General Oronzo Doria's alleged involvement in the abuses at Bolzaneto did not stand in the way of his promotion from colonel to overall commander of the penitentiary police.

"G8 represented a break with the past for Italy", says Senator Martone. "It was a threshold moment in which the Berlusconi government gave an idea of the stuff of which it is made."

Most Britons would like to believe their own government is made of very different stuff. However, it was not until five days after the raid that any of the victims was visited by a British official. "They were violently assaulted, thrown into prison, mistreated and abused, then thrown out of the country," Parry says. "Then the British consulate finally caught up with them, had a few kind words, and that was it."

A spokesperson for the Foreign Office says: "According to Italian law, detainees have to be seen first by a magistrate before they are allowed consular access and contact with a legal representative." She adds that the Italian authorities did not release a list of those detained until July 23. "As soon as we had that list, we did manage to get access."

However, Matt Foot, another British lawyer who represents two of the victims, argues that, given the detentions "breached every section of what your rights are supposed to be when you are arrested in a European country", this consular effort was not enough. "If it had been their own children, they would have tried a bit harder," he says.

Moth recalls that, five days after the Diaz raid, he was "put in a holding cell with the other protesters, and found that US and Spanish consuls had already been in to see their people".

Peter Hain, then minister for Europe, ventured an opinion that the policing of Genoa had been "over the top". He was soon put right by his boss. According to the official account of a lobby briefing given by Tony Blair on July 23 2001: "It was important to keep a sense of perspective. The Italian police had had a difficult job to do. The prime minister believed that they had done that job."

As Blair prepares to host the next G8 summit at Gleneagles in Scotland in July, the Foreign Office pledges that "lawful protest" at the meeting will be supported. Those not prepared to do that, it adds, will be policed "robustly".

The 93 individuals assaulted and mistreated at Genoa three years ago were peaceful, lawful demonstrators. But, as Parry says, even if those sleeping at the Diaz building had been Black Bloc protesters, "Was that then a proper reaction, to go in and nearly kill people by clubbing them until they are lying in pools of blood?"

Italy's painstaking judicial system means the Diaz and Bolzaneto cases are unlikely to end soon. Defendants - and, indeed, prosecutors - have a right to two appeals, so it could be years before final verdicts are given. Like many of the injured protesters, McQuillan received counselling for months afterwards and was "traumatised to the core" by what happened to him. He is not looking forward to testifying at the trial of the police officers charged with involvement in the Diaz school raid. But, he says, "I think it is important to go to court and look at these men who tried to kill me."

He and others who still carry the scars of Genoa remain determined to exercise their right to demonstrate. Doherty has been at all the recent protests in London against the war in Iraq. "I just felt really strongly about things, so I made myself go," she says, adding that the feeling that something terrible is going to happen to her at these demonstrations is "always there, I am always anxious".

Moth, meanwhile, intends to attend the G8 Gleneagles demonstrations this July. He says, "I want to make a statement by going, that intimidating protesters will not silence us."