George Bush took tea with the Queen on his farewell tour of Europe today as protesters gathered to demonstrate against the US-led war in Iraq.
Air Force One landed at Heathrow just before 2pm on the president's last official visit to Britain before he is due to leave office in January.
His arrival prompted demonstrations at Windsor, London and Belfast, where he travels tomorrow, and a heavy security operation.
The largest demonstration took place at Parliament Square, Westminster, where there were approximately 2,000 to 2,500 demonstrators, Scotland Yard said.
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.
The Metropolitan police said it had mounted a "significant" operation involving 1,200 officers.
Officers had completely blocked off Whitehall, with a metal barrier which they were standing in line behind.
Earlier this evening scuffles broke out between waiting police and protesters. Police pushed forward and began confiscating placards as the group thronged towards them.
Tonight Scotland Yard said that police had arrested 25 people, all of whom were in custody at central London police stations.
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.
"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. Today we have done nothing but negotiate to make their demonstration a success.
"The acts that we have witnessed today are deplorable and cannot be described as lawful demonstration."
Speaking ahead of his visit, Bush urged Britain against setting a timetable for withdrawing from Iraq. In an interview with the Observer, he said: "There should be no definitive timetable."
In an apparent warning to Gordon Brown, he added: "I am confident that he [Brown], like me, will listen to our commanders to make sure that the sacrifices that have gone forward won't be unravelled by drawdowns that may not be warranted at this point in time."
After landing, the Bushes travelled by the presidential helicopter, Marine One, from Heathrow to Windsor for tea, sandwiches and cakes with the Queen.
Television footage of the Windsor visit showed Bush putting what looked like a digital camera into his pocket after taking pictures of the castle. He and Mrs Bush then flew on Marine One into central London for dinner with the Browns.
Tomorrow, Bush is scheduled to meet his old ally Tony Blair in the former prime minister's role as Middle East envoy.
Andrew Burgin, a spokesman for Stop the War, said: "We think Bush is a war criminal. Both Blair and Bush are responsible by duplicity and lying for an illegal invasion of Iraq."
The talks between the president and the prime minister tomorrow are expected to cover the world economic crisis, rising fuel and food prices, and attempts to get oil-producing countries to step up production and ease prices.
Laura Bush will follow her own itinerary, visiting the Charles Dickens Museum in London and touring the Afghan and Burmese collections at the British Museum.
Later tomorrow, Bush will travel to Stormont in Belfast and join Brown for talks with the Northern Ireland first minister, Peter Robinson, and his deputy, Martin McGuinness.
Amnesty International staged a demonstration in the Belfast city centre against Bush's human rights record. Around 50 activists dressed in Guantánamo-style orange prison suits marched on Belfast city hall.