Anti-war protesters have marched down Whitehall to Parliament Square, despite being told the protest was illegal.
Hundreds of anti-war protesters took to the streets in 2005.
The Stop the War Coalition timed its protest to coincide with Gordon Brown's Commons statement on Iraq.
Students, campaigners and trade unions joined the rally in Trafalgar Square, before marching down to Parliament.
The group was told it could not march down Whitehall because of a law passed in 1839 which protects the right of MPs and peers to get to Westminster.
But a last-minute decision to let the march go ahead was hailed by organisers, who said they had struck a "significant blow" for democracy.
Lindsey German, convener of the Stop the War Coalition, said they had repeatedly been told they could not go ahead with the march - but said the authorities had underestimated their determination.
The protest coincided with Prime Minister Gordon Brown's statement to MPs, in which he said the plan was to reduce troop numbers to 2,500 by next spring - depending on conditions on the ground.
Ms German said her message to the government was: "You will never draw a line under this war until you bring all our troops home."
Labour left-wing MP John McDonnell said the attempt to "ban" the protest had been "an unacceptable assault on our civil liberties".
Respect MP George Galloway, currently suspended from the House of Commons after a row about his Mariam Appeal charity and his comments about standards watchdogs, said organisers had won a "significant victory".
Speaking at the start of the protest in Trafalgar Square, he said that Mr Brown saw Iraq as a "photo opportunity" but that it had been a "graveyard for a million Iraqis".
Other speakers included comedian Mark Thomas and Ben Griffin, a former SAS trooper.
Owen George, 21, who was at the protest in Parliament Square, said the demonstration was "amazing".
He said: "They managed to get into the square, which is very good. It's amazing how much freedom people do have in this country."
CND chairwoman Kate Hudson accused Mr Brown of trying to ban the protest - despite promising to extend liberties around the world at the Labour Party conference.
However, a Home Office spokeswoman said the march had not been "banned" and talks had been held to find a way of re-routing protesters.
The Metropolitan Police said it had worked with the coalition to "facilitate" a lawful protest.
A Scotland Yard spokesman said: "Our aim is to balance the right of the Stop the War Coalition to freedom of protest whilst maintaining the right of MPs and peers to conduct the business of either House whilst they are sitting."