Police said it was the UK's biggest ever demonstration with at least 750,000 taking part, although organisers put the figure closer to two million.
There were also anti-war gatherings in Glasgow and Belfast - all part of a worldwide weekend of protest with hundreds of rallies and marches in up to 60 countries.
They came as UK Prime Minister Tony Blair, in a speech warning of "bloody consequences" if Iraq was not confronted, directly addressed those marching.
UPDATE via Paul Waugh
Protestor at #iraqinquiry complains re police chaos: "If you're going to have a police state, at least organise it properly."