With gusty winds buffeting their placards, they set off from Hyde Park determined to make their anger clear to US President George W. Bush and British Prime Minister Tony Blair.
"Anti-Bush! Anti-Blair! Anti-war, everywhere!" shouted those at the front of the procession -- estimated by police at 25,000 -- as they streamed towards London's most famous square.
Several blocks away, police arrested two Greenpeace activists who had scaled Big Ben, the landmark clocktower of the Houses of Parliament, at dawn and unfurled a banner that read: "Time for the truth."
Harry and Simon Westaway, brothers in their 20s, were led off to a police station "on suspicion of causing criminal damage" after strong winds forced them to abandon their plans to spend the whole day atop Big Ben.
Police -- already on high alert against a potential terrorist attack -- said they would review security in the wake of the stunt. But they insisted that at no time were the duo in a position to break into the premises.
The march in London was among several planned in major cities in Europe and around the world to mark the first anniversary of the Iraq war and to demand a swift end to Iraq's occupation.
Mindful of the Madrid train bombings nine days earlier, organisers called a moment's silence and released thousands of black balloons at Trafalgar Square in memory of both Iraq war victims and those who died in the Spanish capital.
Jeremy Corbyn, a dissident member of Blair's governing Labour party, pointed to last weekend's election upset in Spain.
There, he told the rally, the Spanish people "listened to the lies of their prime minister (Jose Maria Aznar) and emphatically threw their government out of power".
In a message read out to the protesters, London Mayor Ken Livingstone -- a familiar face at past anti-war events -- said: "Everything we have learnt since the war has told us that it was totally unjustified."
In numbers, Saturday's march was far smaller than the million-strong march in the British capital in February last year, when many felt it was still possible to avert the conflict.
It also paled against the 100,000 to 200,000 who protested in Trafalgar Square last November when Bush was in London for a state visit.
But demonstrators told AFP they felt as strongly about the war as ever.
"I think you'll see from this demonstration that the war is still the central issue in British politics," said London artist Leon Kuhn, 49, who was selling his own postcards depicting Blair as Bush's computer mouse.
"You know the phrase, 'Dead man walking'?" he asked. "That's Blair at the moment."
"There's no doubt that Saddam Hussein was a dictator," said May King, 30, another Londoner, who was helping to carry an effigy of Bush and Blair festooned with dollar bills, oil company logos and silver rockets.
"But was he a threat to the nation? I don't think he was," she said.
Anti-war independent MP George Galloway, who was expelled from the Labour party last October, called for the June 10 local and European elections to be a referendum on Blair's future.
"We must draw a line through Tony Blair and ensure that he is the ex-prime minister of Great Britain," said the veteran left-winger from Glasgow, who met Saddam in Baghdad seven months before the war.
A YouGov opinion poll for Sky News television, released earlier this week, indicated that Britons remain divided over the Iraq war -- with 48 percent saying it was the right thing to do, and 41 percent saying it was wrong.