US President George W Bush arrived in London today at the beginning of a two-day visit to Britain and Northern Ireland.
The visit - which sparked angry protests in central London - marks the end of the President’s “farewell tour” of Europe.
More than 2,000 anti-war campaigners attempted to have their say tonight while Mr Bush was being entertained in Number 10.
Members of the Stop War Coalition, CND and the British Muslim Initiative staged a noisy protest in Parliament Square in a bid to make their views known one last time before the leader leaves the world stage.
Mr Bush is spending two days in the UK renewing acquaintances and holding talks with Gordon Brown and former premier Tony Blair after spending last week meeting leaders in France, Germany and Italy.
Mr Bush and his entourage flew into Heathrow this afternoon on the Presidential jumbo jet Air Force One and were ferried by helicopter to the Queen's Berkshire home.
Mr Bush is the first President to be hosted at the Royal residence since Ronald Regan was pictured riding at Windsor with the Queen in June 1982.
Later on this evening, as Mr Bush met Mr Brown in Downing Street, demonstrators shouted: "George
Bush: terrorist," and brandished placards emblazoned with his name, blood patches and handcuffs.
To loud applause, left wing veteran Tony Benn was among those who addressed the crowd.
He said: "This is a campaign not just about Bush and Blair, although they are war criminals, it's a campaign to get our troops out of Afghanistan and out of Iraq and to stop the threats against Iran and to free the Palestinian people."
Security was tight with scores of armed policeman and rows of police vans - but scuffles still broke out.
Officers blocked off Whitehall, with a metal barrier which they were standing in line behind.
Demonstrators pushed forward and police brandished batons, before riot police were also brought in.
Deputy Assistant Commissioner Chris Allison said the Met Police's "significant" operation would see a thousand officers deployed on the streets of London today and tomorrow to maintain order.
He added tonight: "We are seriously disappointed by the irresponsible and criminal action of those who have challenged police, physically ripping apart security barriers with the sole aim of breaching security in Whitehall.
"London has been at a severe level of threat from terrorism for a number of years and, in the current climate, attacks such as the ones we have seen today are totally irresponsible as such behaviour can be used as cover for more sinister criminal activity."
Later tomorrow Mr Bush will travel to Stormont in Belfast and join Mr Brown for talks with Northern Ireland First Minister Peter Robinson, and his deputy Martin McGuinness.
Ahead of his arrival, Mr Bush urged British Prime Minister Gordon Brown not to set a timetable for the withdrawal of troops from Iraq.
He said in an interview with Britain's Observer newspaper that the United States and Britain, Washington's main ally on Iraq, both obviously wanted to bring their troops home but this could only be "based upon success".
"Our answer is: there should be no definitive timetable," said Mr Bush, adding he was "appreciative" that Mr Brown was in frequent touch about "what he and his military are thinking".
Only about 4,000 British troops are still in Iraq, most of them stationed at a base in the south.
Last week media reports said Britain could possibly pull all its forces out by the end of the year, but with the situation still unstable on the ground that appears unfeasible.
Mr Bush has a more formal relationship with the British leader than with Mr Brown's predecessor Tony Blair, Washington's staunchest supporter over Iraq. Mr Brown is battling against poor opinion poll ratings and Iraq is a divisive issue in Britain.