Police estimated the numbers marching at 110,000. But Chris Nineham, a spokesman for the Stop the War Coalition, said that 350,000 had joined the protest.
"We've shown the warmongers that far from disappearing, we're still growing," he said, "and we'll stay on the streets until we win".
The anti-war MP Alice Mahon said she thought a quarter of a million had taken part. "I've lived through the Vietnam demonstrations, the protests against Thatcher and the Poll Tax," she told Aljazeera.net, "and this is one of the biggest I've ever seen.”
Inevitably, the bombing of the British consulate and HSBC bank in Istanbul cast a pall over the carnival-like proceedings, but protesters were not deflected from their goals.
"Everybody deplores attacks on civilians like this," Mahon said. "They are indefensible but the invasion and occupation of Iraq have increased the risk of terrorism and made the world a more dangerous place. What's needed is a different foreign policy.”
A lunchtime press conference by visiting US President George Bush and British Prime Minister Tony Blair, in which they said the bombings justified the so-called “war on terrorism” was treated with universal derision by those marching.
“The reason we're marching today is because of all the people who were killed," said Carlito, a Huddersfield student who held up a giant skeletal puppet draped in a star-spangled banner.
"Terrorism is a result of oppression, and since September the 11th, George Bush has been oppressing the whole world, including us."
As dusk fell over Trafalgar Square, a huge cheer bounced off the embassies and galleries when a golden six metre high statue of Bush was scaled and toppled by activists in white boiler suits.
Some protesters said it was a
demonstration for US values
On the square's southern flank, two bonfires lit the night sky as protestors burned British and American flags.
“BBC, ITV, this is not for you to see,” they chanted as cameramen and photographers scrabbled for pictures. But American demonstrators were not intimidated by the gesture.
"It's not a tactic that I'd use but the American constitution defends flag-burning and people should have the right to express themselves in that way," said Mitra Abdulahi, a student, 23, from Los Angeles. "This is a demonstration in support of real American values.”
Sanjay Pinto, from Bowling Green, Ohio, agreed. "There's sure a lot of Americans here for an anti-American protest," he said. "People can distinguish between being anti-Bush and being anti-American.”
Massive demonstrations of this kind are becoming a part of the political furniture in western European countries. Some activists said they were frustrated by the police lines preventing them from entering the Mall which leads to Buckingham Palace.
Even so, the mood was one of jubilance and celebration. In a continuation of the creative mood that has characterised the anti-globalisation movement, many protesters carried their own placards.
"I've lived through the Vietnam demonstrations, the protests against Thatcher and the Poll Tax...and this is one of the biggest I've ever seen”
"I'm here to kill Bush (shoot me)" read one; "I never wanted war no matter what TV told me," another.
"It's really jovial here," said Mudasser Hussein, a pharmacist, 25, from High Wycombe said. "It's nice to see so many people who haven't been duped into believing that all Muslims are terrorists.”
Indeed, families, pensioners and people in wheelchairs all coursed through Trafalgar Square in a display of the anti-war movement's sheer diversity.
"I'm here to protest against Bush's policies," said Patrick Jones, 14, from Wolverhampton. "He's not welcome here.”
In the run up to the demonstration, newspaper reports had suggested that many schools were preparing to clamp down on pupils coordinating a strike day. But the attempt did not appear to have been successful.
"This is an educational experience," his mother, Christine, chipped in. "If the school doesn’t like it, tough."