Guerrilla gardeners carried the seeds of destruction

They came on bicycles, they came with babies in prams, they came with their hair painted green. And, also, some came looking for trouble.

They came on bicycles, they came with babies in prams, they came with their hair painted green. And, also, some came looking for trouble.

This was to be the day of direct action, which would constitute nothing more threatening than planting flowers outside Westminster. And for thousands of people this was the way they spent the May Day bank holiday.

But as some protesters chose to clear up the streets behind them last night - bagging up their cans and bottles, placards and piles of horse manure - others were involved in skirmishes with police across central London.

It all started peacefully. A flier advertising the day of "Guerrilla Gardening" organised by the group Reclaim the Streets had promised: "[This] is not a protest: by its very nature it is a creative peace for celebration of the growing global anti-capitalist movement."

The flier explained to people how they should set about transforming the square outside the mother of all parliaments into a new "urban transformation location".

The idea was simple: find a space, plant some seeds, play some music and read some poetry. And when you got bored it would be time to move on to the next empty space ripe for such basic alteration.

Legal observers would be on hand to monitor the police response and there was advice on what to do if arrested. "At all times, they will be trying to get information out of you," it warned. "However friendly they seem, don't be fooled."

Up to 5,000 people of all ages and backgrounds accepted the invitation, armed with nothing more sinister than trowels and seed packets.

As Big Ben sounded11.30am, a party of activists arrived in the square wearing costumes designed to represent the pagan origins of May Day. A woman, naked but for a tiny pair of turquoise pants, followed a group of drummers around the square as climbers strung banners between lampposts. One of the protesters, Andrea, 20, a music student, said: "The general theme is that people want to have something of a say. No one has any free land any more. It's always owned by somebody else."

The politics were as mixed as the colour of the protesters' hair. Some focused on environmental issues, some chose to highlight the alleged role of the World Bank in creating debt in the developing world, others saw it as a chance to rail against capitalism's iniquities.

For most of the day, the protest was shot through with good humour. Graffiti daubed on a statue of Field Marshal the Viscount Slim outside the Ministry of Defence in Whitehall read, "Fat Boy".

The Viscount's crotch had been adorned with suggestive balloons and the slogan "We did it for our country" had been sprayed on to the statue. The smell of marijuana drifted down Whitehall.

But, perhaps inevitably, for some of those gathered, peaceful protest was not enough. Violence erupted in the early afternoon as a group split from the main body of the demonstrators and smashed their way into a McDonald's restaurant. As they did, they hurled bricks and bottles at police. A police commander said three of his officers were hurt after being attacked with "missiles, iron bars and scaffolding".

Deputy Assistant Commissioner Mike Todd of the Metropolitan Police said the rioters "were a minority but they were thugs, not demonstrators. It's an absolute disgrace.

"I know of one police officer who had a brick thrown in his face and others who were beaten to the ground," he said. "The staff in the McDonald's, which was completely destroyed, must have been terrified as they were there just trying to earn a living."

The response to the violence from the police - who had spent weeks preparing for one of their biggest operations for decades - was sharp and officers wearing riot gear moved to close off Whitehall, stopping the demonstrators from joining others in Trafalgar Square.

Gradually, the lines of officers, backed by mounted police, drove the protesters back to Parliament Square where they stood trying to goad the police. As they retreated it became apparent that the Cenotaph and a statue of Winston Churchill had been defaced.

The actions of the violent minority clearly upset most of the demonstrators. One man, his hair painted fluorescent green, kept shouting at those throwing bottles to stop. "What good does that do?" he kept shouting. "Someone is going to get hurt."

While there were only isolated incidents of people being injured, the violence soured the mood. "There are people with families here," said Rachel, 27, a supply teacher. "The only thing I don't like is that you always think there will be violence. You cannot deny that there is a tiny element that wants to cause violence."

For more than an hour, police corralled people inside Parliament Square, blocking off all exits. The intention had been to check to see if any of them could be identified as taking part in the earlier violence, police said.

But that tactic appeared not to have been followed. At around 5pm police allowed people to leave the square and many swarmed down Millbank. Others were understood to be heading for the Millennium Dome at Greenwich, but there were no reports of them arriving. However, skirmishes with police in The Strand, Charing Cross and on Lambeth Bridge were reported as crowds headed for Kennington Park.

By yesterday evening Parliament Square was largely empty and many of the protesters were clearing up rubbish that had been dropped. The "gardens" in the central square were still in place while some pieces of excavated turf remained on the road.

One demonstrator, who gave his name as Good Geoff, said: "I think the underlying consciousness of people is changing. People think that they can have a say. Each person counts more. There is no more hope for politicians."