The campaigners gathered in Trafalgar Square
Organisers estimated that 100,000 had marched from Hyde Park to Trafalgar Square for the event, although the police say the numbers were nearer 15,000.
Marches have taken place in Glasgow, Australia, western European capitals and even the US since the Allies' bombing campaign began.
The Muslim Parliament of Great Britain have joined CND and the Socialist Workers' Party in supporting the anti-war movement.
War aims
A small number of MPs have also voiced their opposition to military action in the House of Commons.
Ex-Labour MP Tony Benn told the rally he hoped Sunday marked the start of a movement for world peace.
He said parliament was "passive" and Tony Blair's cabinet was "cringing" in its failure to question the UK's role in the bombing campaign.
Stop the War committee member Mike Marqusee said he was delighted with the "very diverse" turn out.
"We are hoping to send out a clear message that Tony Blair does not speak for Britain and we hope that message carries across Britain and across the world," he said.
"We believe that this march reflects both the scale and diversity of anti-war opinion in this country.
"Our campaign will not end until the war ends and Britain and the US stop the bombing."
'Tyranny'
Labour MP Alan Simpson told the crowd: "When people say 'the bombing has won, the Taleban have been driven out', I say that it is not the answer at all.
"We have seen the removal of one feudal tyranny, only for it to be replaced by another."
Fellow Labour MP Paul Marsden accused Tony Blair of being "drunk with power" in his handling of the war on terrorism.
He told the rally: "You are sending another powerful message to Number 10 and to the White House that we are not simply going to allow the atrocities of September 11 to be replaced with further atrocities in Afghanistan."
Organiser Lindsey German said the bombing campaign had done nothing to tackle international terrorism.
"The war aims were never to install the Northern Alliance into Afghanistan to replace the Taleban.
"Most people who know anything about both regimes regard the Northern Alliance as just as bad.
"Meanwhile terrorism continues and will continue because the real roots of terrorism have not been dealt with and are not being dealt with in this war."