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.

I thought I needed to show that we were against the war so the prime minister can't say that he has the backing of his people Francesca Morrison

Click here for pictures and more opinions from the march

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.

He did not "seek unpopularity as a badge of honour", he said, "but sometimes it is the price of leadership and the cost of conviction".

Shortly after he spoke, at around midday GMT, a tide of banner-waving protesters began surging through central London.

	:	Anti-war protests
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They cheered, shouted, sounded horns and banged drums, waving signs with slogans 'No War On Iraq' and 'Make Tea, Not War'.

Contingents arrived in the capital from about 250 cities and towns across the UK.

The three-and-a-half mile march - organised by Stop the War Coalition, the Campaign for Nuclear Disarmament (CND) and the Muslim Association of Britain - was started early by police, over concern at the number of people gathering.

Two separate meeting points were used before the streams converged in Piccadilly Circus and made their way to Hyde Park for a rally.

Christian message

Organiser John Rees said the turnout had been fantastic with an "electric atmosphere but also very serious and determined".

Leading the demonstrators into the park was Italian student Giancarlo Suella, 29, who held a banner reading: 'Bush And Blair, A Good Christian Will Never Kill'.

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He said: "I came to England to make my point to Mr Blair, it's hard to believe what he is doing."

All police leave in the capital was cancelled for the event but Scotland Yard said it passed off almost without incident.

There were a handful of arrests for minor mostly public order offences, but later four anti-war activists were arrested after more than 20 people held a sit-down protest at Piccadilly Circus.

The protesters - who were part of the Voices in the Wilderness UK pressure group - were taken to a local police station and the road was reopened at 2015 GMT.

Andy Todd, assistant deputy commissioner of the Metropolitan Police, said the crowd had been tolerant and patient and "the biggest I have experienced."

The police estimate of 750,000 people could be an underestimation due to people bypassing official routes or going straight to Hyde Park without joining the main march.

At the rally, Liberal Democrat leader Charles Kennedy told the crowd he was not persuaded by the case for war.

With "misleading" evidence provided by the government, "it's no wonder that people are scared and confused", he said.

High profile speakers

Former US presidential candidate the Rev Jesse Jackson also spoke and led the crowd chanting "give peace a chance, keep hope alive".

Among other high-profile supporters were writer Tariq Ali, ex-minister Mo Mowlam, London's mayor Ken Livingstone, actress Vanessa Redgrave, human rights campaigner Bianca Jagger and former MP Tony Benn.

Playwright Harold Pinter made a rare public speech, saying America was "a country run by a bunch of criminal lunatics with Tony Blair as a hired Christian thug".

Hollywood actor Tim Robbins, also attending, told BBC News the crowds were "what democracy looks like".

If Mr Bush and Mr Blair ignored them "they are not rightful leaders of a democracy", he said.

There was one gesture of support for military action to remove Saddam Hussein elsewhere in London during the rally.

Writer Jacques More, 44, from Croydon, south London, stood with a placard outside the Iraqi section of the Jordanian embassy in central London, saying that although a last resort war was necessary "when evil dictators rule and murder their own people".