Thousands of peace demonstrators marched through the streets of central London on Saturday, waving banners saying "No Occupation of Iraq."
Many marchers also brought flowers, cards, wreaths or whatever they felt appropriate to lay outside 10 Downing Street as they walked past.
The Stop the War coalition, the key organizer of peace marches in Britain, estimated some 100,000 protesters joined the peace march, but Scotland Yard put the number at nearly 20,000.
However, former Pakistani cricket captain Imran Khan, who joined the march, said the numbers were irrelevant.
"It doesn't matter how many people turn out, it's about registering a protest that a principle has been violated, international law has been violated and everyone who cares must register a protest," he said.
Anti-war campaigner Chris Nineham said he believed "a great deal more problems" lay ahead for the British and US forces as they tried to take over Iraq's administration.
"I don't really believe the fighting is over, I think the invasion is sliding into a colonial occupation," he said.
The march, called by Stop the War with the Muslim Association of Great Britain and Campaign for Nuclear Disarmament, is the third mass London rally to be held.
Hundreds of anti-war protesters also took to the streets of Glasgow, in a march organized by the Scottish Coalition for Justice not War.
On Saturday, anti-war protests also took place in about 40 other countries, including New Zealand, Japan, South Korea, Italy,Greece and France.
Andrew Burgin from the Stop the War Coalition said there was a fear that Iraq was only the beginning in a series of wars planned by the United States, and possibly Britain.
"Iraq now, but will it be Syria and Iran tomorrow?" he asked.