George and Laura Bush are on a two-day visit to the UK. They arrived at Heathrow airport on Sunday on Air Force One, after spending time in France.
President Bush was greeted on the airport runway by the US Ambassador to Britain, Robert Tuttle.
At Heathrow, security was tight. Armed police stood close by as the president and first lady arrived.
Mr Bush went straight from Air Force One to the presidential helicopter, Marine One, for the short flight to Windsor where he had tea with the Queen.
The Queen gave Mr Bush a tour of St George's Hall in Windsor Castle. Mr Bush is the first US president to visit Windsor since Ronald Reagan in 1982.
Police sealed off Whitehall in London in anticipation of Mr Bush's visit to Downing Street.
Shortly before the US president met Prime Minister Gordon Brown hundreds of anti-war protesters gathered in Parliament Square, where there were a few scuffles with police.
Mr Bush was greeted at the doors to 10 Downing Street by the prime minister's wife Sarah. The president and his wife Laura dined with the Browns.
The two leaders meet formally on Monday when they are expected to focus on the Middle East, fuel and food prices.