Thousands of protesters gathered for an anti-war rally in London calling for an end to the US and UK occupation of Iraq.
Saturday's march was addressed by leading anti-war activists
The march started in Hyde Park and was followed by a rally in Trafalgar Square addressed by speakers including Tony Benn, filmmaker Ken Loach and Labour MP George Galloway.
The organisers said they wanted to make Labour respond to their anger over the situation in Iraq, ahead of the party's annual conference.
Police estimated there were about 20,000 marching, although organisers estimated up to 100,000 had attended the event.
Many of the protesters chanted anti-Bush slogans and carried banners with messages like "Blair must go" and "UK troops
out of Iraq".
Others blew whistles and klaxons, making for a carnival atmosphere with a mobile sound system belting out live political songs and numerous drummers playing.
Among the more unusual sights on the march was a replica tank made from cardboard and painted orange, pushed by members of anti-capitalist group Globalise Resistance.
Speaking to the rally, Labour MP George Galloway attacked the prime minister for refusing to discuss Iraq and said a UN force in Iraq would still be "foreign occupiers".
"I want to deal with a very dangerous diversion, a
very dangerous red herring, a false dichotomy which is emerging in this debate - whether or not Iraq should be controlled by a group of foreigners with blue helmets or a group of foreigners with stars and stripes.
"Foreign occupiers are not the solution to Iraq's problem, they are Iraq's problem."
Of America, Ken Loach said: "Never in modern times has there been such a concentration of power and wealth... and never before has a British prime minister shown such supine fawning
in [the President's] favour."
Saturday's march was the fifth protest this year against Britain's participation in the US-led invasion of Iraq, and was organised by the Stop the War coalition, the Campaign for Nuclear Disarmament and the Muslim Association of Britain.
A demonstration on 15 February attracted a record-breaking turnout of about one million protesters, attracting worldwide attention.
Frank Sanders, a 55-year-old town planner from Chelmsford, Essex, said: "I want to see an end to the occupation of Iraq and a return of the allied troops.
"Iraq should be an independent country. This was an illegal attack and it is an illegal occupation."
Ahmed Ahmed, a 40-year-old doctor from London, said: "The causes for war were not true, it wasn't about freedom. I agree that Saddam Hussein was bad but this was not the way to get rid of him."
Tatiana Wells, 29, from Brazil, added: "I am here to raise awareness in Brazil where we think the war has finished. We need to spread the word that the war is still going on."
The Metropolitan Police said 2,700 officers had been drafted in to cope with the rally but it had passed off without any arrests.
In an anti-war demonstration in Edinburgh on Saturday, police estimated about 1,500 people attended a simultaneous march and rally, while a similar number were reported to have protested in Dublin.
About 3,000 demonstrators marched through Paris, while in Athens a further 3,000 confronted riot police outside the American embassy.
In the South Korean capital, Seoul, more than 2,000 people marched on the city centre to oppose the sending of South Korean troops to Iraq.
There were also protests in Vienna, Berlin, and Madrid, as well as New York and San Francisco.