Simon Chapman, an anti-globalisation activist, is in a critical condition at the hospital wing of the Korydallos maximum-security prison in Athens. He has protested his innocence by refusing all food since early October.

The 30-year-old, from Basildon in Essex, was among seven protesters charged with possession of explosives and weapons after violence between Greek riot police and demonstrators marred the final day of the summit. Five of the seven, including two Spaniards a Syrian and a Greek, are also refusing food.

This week, the prosecutor in Greece's Supreme Court took the unexpected step of ordering an investigation into the circumstances of Mr Chapman's arrest. The television footage that lawyers say shows the charges to be groundless, has increased the pressure on Greek authorities to allow the hunger strikers bail.

But friends and relatives fear any decision will come too late to save him. "Every day and every hour is important now because Simon is in great danger," his French fiancée, Mylan, who withheld her surname, told The Independent. "He is innocent and the court needs to make a decision."

Mr Chapman's lawyer, Haris Ladis, said police and the authorities had been playing a "dirty game" with his client. The tapes, from two Greek channels, show Mr Chapman clearly wearing a blue rucksack during fighting with police. Separate footage then shows Greek police in riot gear collecting petrol bombs and other items into a black rucksack. Mr Chapman has been charged with possession of a black rucksack containing petrol bombs, a hammer and an axe.

Mr Ladis said: "The video shows the scene where Mr Chapman was detained wearing the blue rucksack. A couple of minutes later you have him sitting down after being badly beaten by the police and there is a new rucksack, which is black." The lawyer said his client was "on the point of no return". He added: "Even if he starts emergency treatment there may already be irreparable damage."

But the Greek judicial system is slow. No trial date has been set and, under Greek law, suspects can be detained for up to 18 months. Another defence lawyer, Vasso Karaindrou, said the constitutional rights committee of the Athens bar, of which she is a member, had filed a second application for the release of the seven. Their detention since June has brought protests by anarchists and leftists across Greece, and weeks of minor bombing attacks in the capital.

Ms Karaindrou contrasted their treatment with that of Vladimir Gusinsky, the former Russian media baron who was bailed within just four days of his arrest in Athens in the summer. "It is strange to see that the system in Greece has different standards," she said. "None of [the seven] has a record nor any other outstanding warrants." A Thessaloniki appeal court is expected to decide next week whether the hunger strikers can be released pending trial, raising the prospect that prison authorities may force-feed the hunger strikers if their condition deteriorates.

A third defence lawyer, Yiannis Yeropoulos, said: "If there's no decision in two or three days, force-feeding will become a possibility. There's no clear answer on this issue, they can force them and that is what has been done in the past but it's not legal." Amnesty International considers feeding prisoners against their will is a form of torture.

Mr Chapman, before his transfer to the prison hospital said he felt the police had been looking for "someone to punish" when they arrested him. "I don't think it's anything personal against me directly. I think they felt someone had to be punished for the disturbances on the day and it didn't really matter who it was as long as they had someone."

Mr Chapman's brother, Chris, said Simon was determined to see the strike through to the end.

"I am very concerned," he said. "He's obviously lost a lot of weight since 5 October and his health is obviously deteriorating rapidly. He's determined to continue his hunger strike until the situation is resolved."

Simon's fiancée, Mylan, who is in Athens campaigning for his release, said the hunger strikers were suffering unjust treatment. "I am allowed only two visits a month," she said. "I've seen Simon only seven times since the arrest."

On Thursday Greek students took over the dean's office at Athens University to protest at the imprisonment and a march in Athens called for their immediate release.