More than 2,000 demonstrators rallied in central London to protest US president George W Bush's visit at the end of a European farewell tour, police said.
"George Bush: terrorist," shouted the protesters, gathered peacefully in Parliament Square a couple of hours before Bush was due to dine with Prime Minister Gordon Brown in nearby Downing Street.
Earlier, protesters demonstrated outside Windsor Castle, south-west of London, where Bush started his British stay in talks with Queen Elizabeth II.
About 1,200 police were deployed or on standby in central London, said a Scotland Yard spokeswoman, who put the number of protesters at between 2,000 and 2,500.
The demos were "noisy but peaceful" and no incidents were expected, but they had planned for every eventuality, she told AFP .
Scores of policemen, some armed, were backed by rows of police vans on Whitehall, the main government thoroughfare which links Parliament Square to Trafalgar Square.
The rally was organised by the Stop the War Coalition, the Campaign for Nuclear Disarmament (CND) and the British Muslim Initiative.
"George Bush is on his way out," said Paula Mitchell of the Socialist Worker Party, which is part of the coalition. "He as an individual is not that significant but it's what he represents.
"Whoever replaces him in the US is going to carry on with the same policy. They're preparing to install themselves in Iraq semi-permanently. It's turning Iraq into a colony while thousands of people continue to die."
Nigel Green, wearing a Give Peace a Chance badge and giving out Socialist Worker leaflets, told AFP: "Obviously we are all looking forward to the day he leaves office.
"But I think that it's not guaranteed that the American and British troops are going to leave Iraq or Afghanistan." Asked if anger over Iraq had calmed, he said: "No, if anything it's as bad as it ever was."
Veteran protester Brian Haw, who has manned a permanent peace protest in Parliament Square for over seven years, was greeted with whistles and cheers when he went on stage.
"What is going on is so inhumane," he told the demonstrators. "How are we ever going to clean up this mess?" he asked, to applause.