Thousands of protesters from Sussex joined what organisers described as the largest ever peace demonstration in the UK.
Organisers said more than one million people had joined the anti-war rally in London on Saturday to voice opposition to a conflict with Iraq.
Hundreds of thousands of campaigners crammed into Hyde Park to hear a succession of speakers, including former US presidential candidate Reverend Jesse Jackson, former Labour Cabinet minister Mo Mowlam, Liberal Democrat leader Charles Kennedy and veteran campaigner Tony Benn, take to a specially erected stage near Speakers' Corner.
Waving flags and banners demanding No War On Iraq and Make Tea, Not War, protesters cheered and shouted as speakers put across the case against war.
The turnout, which brought the city to a standstill as more and more people joined to swell the numbers further, dwarfed last year's 400,000-strong countryside march.
More than 28 coaches left Brighton on Saturday morning with hundreds more people travelling from Brighton, Hove and elsewhere in Sussex by train.
They were just some of the 1,000-odd coaches to travel to the capital to ferry protesters in from all over the country on a day of protests around the world.
The march in London was organised by Stop the War Coalition, the Campaign for Nuclear Disarmament and the Muslim Association of Britain which organised similar rallies in Glasgow and Belfast.
City councillor Linda Austin, who carried the Brighton Labour Party banner on the march said: "I believe very firmly that it is wrong to go to war now."
Former Brighton and Hove mayor Francis Tonks said: "If there is a war it will spark off tremendous international conflict, which I think will harm our country and inevitably it will be interpreted as an attack on the Muslim world."
The Prime Minister's office said in a statement that Tony Blair "respected the views" of those taking part in the march but suggested the gesture would make little difference to Government policy.
"The way to prevent any military conflict is for Saddam Hussein to co-operate and fulfil his obligations as set out by the whole of the international community as embodied by the United Nations," Mr Blair's official spokesman said.
"We have to pursue international justice through the United Nations."
The march took place the day after chief weapons inspector Hans Blix told the UN that no weapons of mass destruction had been found in Iraq, but forbidden materials remained unaccounted for.
Peace protesters in Brighton sent a Valentine's Day message to Tony Blair and George Bush in advance of the anti-war rally in London.
Some 200 campaigners gathered outside Churchill Square in Brighton at sunset on Friday insisting there should be no attack on Iraq.
Cath Senker, of Sussex Action for Peace, said: "We are putting across a Valentine's message to make love not war.
"Bush is determined to go to war whatever. They have made their plans, they want to go ahead but I think we can actually stop it."
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