Peace campaigners have marched on the US Embassy in London demanding an end to the war against Iraq.
MP George Galloway was among Saturday's war protesters
Organisers estimated that about 3,000 demonstrators joined the march, while the Metropolitan Police put the figure at about 1,000.
It is the first time the US Embassy, in the centre of the city, has been targeted by protesters since the war began.
Organised by the Stop The War Coalition, the protesters met outside the BBC's Broadcasting House before marching to the embassy in Grosvenor Square, where there was a heavy police presence.
Six people were arrested during the demonstration for public order offences.
Demonstrations also took place at military bases, including RAF Fairford in Gloucestershire, where B-52 bombers are based, as has happened since war began.
When I see the pictures of Iraqi people drinking sewage water, I also see
the coalition dropping bombs on them
Stop The War Coalition
At the London rally, Stop The War coalition member Asad Rehman said those politicians who had voted for military action had "the blood of the Iraqi people on their hands".
Mr Rehman told protesters: "There is only
one person who should be in prison from this war and that is Tony Blair.
"I see bombs continuing to fall on innocent people of Iraq.
"When I see the pictures of Iraqi people drinking sewage water, I also see
the coalition dropping bombs on them."
Costs of war
Peace campaigners on the march urged people to demonstrate outside town halls on Wednesday - Budget Day - when a protest is also planned outside Parliament.
Anti-war campaigners will be railing against the
£3bn cost of UK involvement in the conflict.
American anti-war activist Matt White told Saturday's crowds in London: "George Bush, if you can hear this, you have said if the people are not with you and with the coalition, they are the enemy...
WAS WAR RIGHT?
Source: YouGov online poll
"Look out here and see the enemy today."
Marchers were hemmed in by temporary barricades as they listened to the speeches, before being allowed to disperse.
Poll war support
The protest came as a new online poll by YouGov suggested 68% of British people thought Tony Blair's handling of the war had been excellent or good.
Of the 3,800 adults questioned on 3/4 April, 28% said his handling had been poor or very poor.
The poll, for ITV's Jonathan Dimbleby programme on Sunday, showed 55% of those surveyed said the US and UK were right to have taken military action against Iraq, with 38% saying they were wrong.
On Saturday, Bedford was another town where anti-war campaigners staged protests, with organisers saying about 500 people attended the rally.
Elsewhere, about 120 protesters from the Portsmouth Resistance Group reportedly used fake blood to stage a "die in" at the city's naval base.
Richard Byrne, a spokesman for the Reclaim the Bases campaign, said about 100 protesters were carrying out a "sponsored stop and search" at the Fairford base this weekend.
The idea was to see how many times demonstrators could be stopped by police, who have a special order under the Prevention of Terrorism Act to search people in the vicinity of Fairford, he said.
Dave Cockroft, of the Gloucestershire Weapons Inspectors Group, said the event was never planned as a formal protest.
But the police seemed had determined to supervise it as a demonstration, he said.
A police spokeswoman said there had been minimal disruption so far.
"We have stepped up the police presence because of the well-publicised possibility of protests," she said.
Devonport naval base in Plymouth and RAF Stafford were among other targets for protesters on Saturday.
Demonstrations have taken place throughout the 16-day military campaign, although none has quite reached the massive crowds which gathered for a London rally in February.
Saturday's protests may be dwarfed by anti-war protests due to take place in 34 major cities across the globe on 12 April.