As Prime Minister Tony Blair struggled to shake off fierce UK criticism of the invasion on Sunday, organisers said between 65,000 and 75,000 protesters had taken to the streets for the peaceful march.
"Troops out," screamed one of many placards being waved by demonstrators. "Blair must go," said another.
Sunday's march was the latest in a series of demonstrations organised by the Stop the War Coalition before and after the US-led invasion of Iraq in March 2003.
The invasion was backed by the British government but only a minority of the electorate.
The march was arranged to coincide with the end of the three-day European Social Forum held in London.
It also comes after a stormy week for Blair, who was accused in parliament last Wednesday of misrepresenting intelligence on Iraq to make the case for war.
One of the brothers of Kenneth Bigley - the British captive recently beheaded in Iraq - had urged people to turn out in force for Sunday's march.
Thousands of European activists
also joined the march
"For Ken's sake and for the sake of everyone in Iraq I ask you to make your feelings known to our government, to protest and to join the demonstration," Paul Bigley said.
The latest demonstration came after 25,000 protesters marched through London in March on the first anniversary of the Iraq invasion.
On that occasion, two protesters scaled Big Ben at dawn and unfurled a banner that read: "Time for the truth."
Last November, up to 200,000 people protested in Trafalgar Square when US President George Bush was in London for a state visit.
Before the Iraq war in February 2003, police estimated that one million people descended on the capital to protest against the looming invasion, while organisers said the figure was nearer two million.
Sunday's protest came just days after Blair apologised to parliament for flawed intelligence on Iraq. But Blair, gearing up for a general election expected next year, angrily denied charges that he "misrepresented" the intelligence to make the case for joining the US-led invasion last year.
Nevertheless, the march took place amid speculation that Britain was to agree to a US request to redeploy its troops in Iraq. A defence ministry spokesman said Defence Secretary Geoff Hoon would brief parliament on Monday following reports.
"He plans to make a statement to the House [of Commons] tomorrow. What he is going to be saying is 'we have been approached by the Americans to deploy British troops in their area of operations'.
"He will also be stressing that no decision has been made and that we continue to consider their request and will do so on its individual merits. He won't be naming units, he won't be giving you a start date or anything like that," the spokesman said.