According to the organisers of the demonstration, Stop the War Coalition and the Muslim Association of Britain, 100,000 people attended the march and rally, which ended in Trafalgar Square in the heart of the capital.
Police said up to 15,000 attended the demonstration, which occurred on the eve of both the governing Labour Party’s annual conference and the anniversary of the outbreak of the al-Aqsa Intifada on Sunday.
With scores of Palestinian flags fluttering amid a sea of antiwar placards, students, mothers with pushchairs and pensioners marched side by side on a crisp autumn day to protest against Prime Minister Tony Blair’s decision to back the US-led invasion of Iraq.
Some protesters held homemade banners calling for "regime change" in London, others chose to dress up as pink fairies holding flowers and one group of protesters pushed a replica orange tank made of cardboard.
Sound systems playing reggae music, whistle-blowing demonstrators and children blowing plastic horns added to the carnival atmosphere.
The event in London coincided with a day of protests across the world. Demonstrators marched in several European cities from Berlin and Madrid to Vienna and Athens. Elsewhere, there were rallies in Turkey, Lebanon and South Korea. New York and San Francisco led protests in the US.
Peace activist Siama Ahmed, who recently returned from Palestine, said she was furious with Blair.
"It really makes me angry to know that our Prime Minster backed Bush into launching an illegal war against defenceless Iraqi civilians. We may not have been able to stop the war in Iraq, but I’m here to tell Mr Blair that his days of being in power are numbered."
"There are no weapons of mass destruction in Iraq, this war is about oil and about spreading American imperialism"
Jenny Saunders, a nursery nurse from Leeds added: "We’ve been lied to: there are no weapons of mass destruction in Iraq, this war is about oil and about spreading American imperialism."
Mustafa, a Palestinian student visiting the UK said: "This is a brilliant demonstration. I’m very happy to see so many people here and I’m touched to see people flying my country’s flag. This is a day that I will never forget."
War a ‘disaster’
By 3pm protesters began to file into Trafalgar Square to listen to speakers including filmmaker Ken Loach, Labour MP George Galloway and former Labour minister Tony Benn, who said that the Iraq war had been a disaster.
"I’m very happy to see so many people here and I’m touched to see people flying my country’s flag"
A veteran socialist stalwart, Benn called on the protesters to hold the British government accountable for its actions in Iraq and to continue to strengthen the antiwar movement in the UK.
This was the fifth demonstration held in the capital since the US led invasion of Iraq.
In February, a record-breaking one million protesters assembled in Hyde Park to hear speakers including the US human rights campaigner and Democrat Reverend Jessie Jackson, veteran Australian journalist John Pilger and leading Muslim activists speak against plans to attack Iraq.