Police have arrested 25 protesters as demonstrations against George Bush's visit developed into a stand-off.

Protesters had been gathering in Parliament Square for their final head-to-head with the outgoing US president.

As the American leader met with Gordon Brown in Whitehall, demonstrators shouted: "George Bush: terrorist," and brandished placards emblazoned with his name, blood patches and handcuffs.

Members of the Stop War Coalition, CND and the British Muslim Initiative organised the rally.

There were approximately 2,000 to 2,500 demonstrators in Parliament Square, Scotland Yard said, with around 1,200 police officers working on each day of the President's visit.

Deputy Assistant Commissioner Chris Allison said: "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."

He said: "In a number of meetings with the Stop the War Coalition we have attempted to work with them to facilitate lawful protest, offering them a number of opportunities to express their views in a legitimate manner. We have done nothing but negotiate to make their demonstration a success.

"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 are totally irresponsible as such behaviour can be used as cover for more sinister criminal activity.

"In this climate any activity to challenge the security of the President of the United States of America will be robustly responded to by police. Police officers faced with people attempting to breach our cordons have no option but to protect them.

"The acts that we have witnessed are deplorable and cannot be described as lawful demonstration."

Earlier, the scenes were quieter as Mr Bush enjoyed afternoon tea with the Queen.

The US leader and his wife Laura sat down with the Monarch and the Duke of Edinburgh for the informal gathering in the lavish setting of Windsor Castle.

Mr Bush is spending two days in the UK renewing acquaintances and holding talks with Prime Minister Gordon Brown and former premier Tony Blair after spending last week meeting leaders in France, Germany and Italy.

Afternoon tea with the Queen is in stark contrast to the serious discussions the President will hold with Mr Brown on Monday.

Mr Bush has already urged the Prime Minister not to jeopardise coalition gains in Iraq by the premature withdrawal of British troops.

The US leader has said there should be no "definitive timetable" for troop reductions.

His comments follow a report last week that the Prime Minister could make an announcement on the final pull out of Britain's remaining 4,500 troops before the end of the year.

Mr Bush and his entourage flew into Heathrow on Sunday afternoon aboard the Presidential jumbo jet Air Force One and they were ferried by helicopter to the Queen's Berkshire home.

Air Vice Marshal David Walker, master of the household, was waiting to greet the US leader as a US Marine opened the door to the military aircraft and Mr Bush walked down the steps.

Security was tight around the landing site in Home Park close to the Royal Palace with armed officers patrolling nearby hedgerows and trees and American secret service personnel stood just beyond the rotor blades of the helicopter.

Mr Bush his wife travelled the short distance to Windsor Castle in a limousine for a private greeting with the Queen in the palace's White Drawing Room.

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.

The Monarch last met Mr Bush during her state visit to the US in March last year. During the tour Mr Bush winked at her after making one of his most famous gaffes.

The Royal party were joined for tea by America's ambassador for Britain Robert Tuttle and his wife Maria.

The group enjoyed traditional English afternoon fayre of tea, small sandwiches and cakes.

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