Last Sunday afternoon in Parliament Square, Central London, and several thousand anti-war protesters were gathered to mark President George W. Bush's last official visit to Britain.

The crowd was, as usual, relaxed, supportive and friendly. So friendly that some people reckon an anti-war demo is a good place to find a partner. It certainly beats internet dating.

The man I was to meet that day, however, left me angry and bemused.

At the start, the demo - organised by Stop The War Coalition, CND and the British Muslim Initiative - was peaceful, if lively.

Many protesters came with hooters, drums, screeching whistles and saucepan lids - every one of them passionately opposed to the invasion of Iraq.

But looming over us ominously was a heavy security presence: riot police, armed officers, even snipers on rooftops.

These days it appears permissible to wave a gun at Britons exercising their democratic rights.

The plan was to walk up Whitehall and deliver a letter to Downing Street, where Bush was meeting Prime Minister Gordon Brown. No chance. We found the road cordoned off with two rows of railings and columns of policemen.

As a throng of protesters built up by the barriers, an extremely animated demonstrator in a white T-shirt caught my eye.

He was near the front screaming abuse at the police and trying to get a friend further back to join him. The second man sheepishly refused his encouragements to edge forward.

The man in the T-shirt was tall, well-built and handsome, smiling but with a hint of menace. He pushed aside children and elderly people.

He continued to shout slogans such as: 'Pigs Out.'

On his back was a black rucksack and he carried a professional-looking camera with a large telephoto lens. Hardly the sort of kit for a few snaps of his day out.

My friends and I, standing a few rows back, asked him a couple of times to calm down, but he ignored us.

I wondered why I was drawn to him. Was it his dark good looks or was I worried for the safety of my 70-year-old friend and children nearby?

Then it dawned on me. I had met this man at a party. I tapped him gently on the shoulder and said: 'Have we met before?'

Instantly he recognised me. 'Hi, how are you? It's really nice to see you here.'

My puzzlement grew. This chap wasn't really the sort you'd expect to see shouting abuse at police officers at an anti-war demo. He was, after all, a policeman himself - and a high-ranking one at that.

I'd met the police inspector at a party around last Christmas. The local mayor was there, along with councillors from other parties and journalists. I'd been asked along by a friend.

Later, we went to a local gay club, where I danced with him and a few others until 3.30am.

He had a bolshie charm, was cocky and a little manipulative. He was also highly entertaining, bragging about his work in the police and how important he was.

I remained bemused about his presence at the demo. I asked if he would send me copies of his demo photos. He replied: 'No, they're to put on my bedroom wall.'

I then casually asked why he was shouting anti-police slogans.

'Funny you chanting that,' I said, 'when you're a policeman.'

They don't have my sort in the police, love,' he said camply, so I would assume he was referring to being gay. A few seconds later, he melted into the crowd.

I wondered whether he was at the demo undercover, deliberately whipping up trouble that he could capture on camera. That would then be used to malign anti-war protesters as dangerous and violent subversives.

Of course, it is possible he was there off-duty to support the anti-war cause, but it is hardly likely he would enjoy chanting slogans against the police.

Equally, he could have been legitimately monitoring the crowds, but again he surely would have been quieter.

I realise there are times when the police need to work undercover if they suspect a crime is being committed. But that is quite different to going into a crowd as an agitator to create disruption.

I went home from the demo feeling furious and did a little research into him. It turns out that he is on more than nodding terms with controversy.

A year ago he advertised himself on the internet looking for sexual contacts with men.

There are plenty of gay policemen - which is to be applauded - but few advertise themselves as such on the internet. It isn't sensible, let alone dignified. Still, I expect the uniform is an extra marketing tool.

I also found out that he is no mere rank-and-file officer.

Last week, it was reported that police at the demonstration had made 25 arrests - including a 60-year-old woman for indecent exposure.

Ten officers suffered minor injuries and two protesters were taken to hospital.

Stop The War has organised about 20 marches in the past, all of which have been peaceful. This is the first where there has been violence.

I cannot say this man was responsible for the trouble, but I saw him try his best to urge the crowd forward.

It is hard not to despair at the remarks of the Metropolitan Police's Deputy Assistant Commissioner Chris Allison, who told reporters: 'We are seriously disappointed by the irresponsible and criminal action of those who have challenged police ... We have done nothing but negotiate to make their demonstration a success.'

But however you look at it, the thuggish behaviour of the man I saw is hardly what you expect of someone employed to protect the public.

Our civil liberties are being eroded daily. The likes of this man are playing a part in destroying the few we have left.

If our security relies on idiots like him creating their own evidence to reinforce fear, who can we trust?

By Yasmin Whittaker-khan
Last updated at 10:20 PM on 21st June 2008
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