Ken Livingstone is due to address a rally against the war in Iraq and advocating a free Palestine in central London this afternoon.
Thousands of protesters came in coaches from across Britain to protest against the US and UK invasion of Iraq earlier this year.
The marchers assembled in Hyde Park at noon, and walked peacefully to Trafalgar Square through Picadilly.
Similar events took place in Paris, Berlin, Tokyo and Washington, as well as numerous other capitals.
Though the London organisers had predicted up to 100,000 in attendance, police estimated around 20,000 had passed along the route by 3.30pm.
There had been no arrests, and officers described the crowds as 'cooperative'.
A spokesperson for the Stop the War Coalition said: "A clear majority of British voters feel according to the latest polls that the war was not justified.
"The death and destruction in Iraq, the revelations of the Hutton inquiry and the continuing failure to find weapons of mass destruction are all fuelling opposition to the war and occupation."
Among other guests due to speak at the rally were film director Ken Loach, Tony Benn, George Galloway MP, school students and speakers just returned from Iraq.
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