George W Bush arrived in London on Tuesday for what the Buckingham Palace described as 'the first state visit by a US president to Britain'.
Prince Charles greeted Bush and his wife Laura at the Heathrow airport, from where they were taken by a helicopter to Buckingham Palace.
Some 14,000 police officers, three times the original estimate, have been drafted in from all over the country to provide protection for the president during his four-day visit.
Fears of a terrorist attack and a number of antiwar demonstrations have forced the Metropolitan Police to undertake an unprecedented security operation, estimated to be costing as much as £10 million.
A Metropolitan Police spokesman said, "Our aim is to balance the needs of security while facilitating any lawful protest and we hope that the protestors who do demonstrate will do so peacefully."
US security official had demanded draconian measures, including the closure of the London Tube [mass rapid transit] network and immunity from prosecution for any of its snipers if they were to accidentally kill a protester.