George Bush has arrived in the UK for the first state visit by a US president amid some of the tightest security London has ever seen.
President Bush and his wife were welcomed by Prince Charles
Prince Charles met Mr Bush and his wife at Heathrow, before they travelled via helicopter to Buckingham Palace for a private reception with the Queen.
The last president to stay in the palace was Woodrow Wilson in 1918.
Scotland Yard has put in place a £5m operation which will see over 5,000 police on the capital's streets.
An opinion poll suggests more Britons back the visit than oppose it, but tens of thousands of demonstrators are expected at a march on Thursday.
PRESIDENT BUSH'S ITINERARY
Tuesday 18 November - Arrives and receives private welcome at Buckingham Palace
Wednesday 19 November - Meets Michael Howard and Charles Kennedy, gives speech on transatlantic alliance and meets UK families of 11 Sept victims before attending royal banquet with Queen
Thursday 20 November - Meets British soldiers who fought in Afghanistan and Iraq, holds meetings with Tony Blair at Downing Street and hosts dinner at US Ambassador's residence
Friday 21 November - Travels to Tony Blair's Sedgefield constituency to meet members of the public before returning to Washington
Officers say security measures reflect the general terrorist threat as well as the need to police the protests.
Despite the huge numbers of police preparing for the visit, grandmother Lindis Percy, 61, managed to climb the front gates of Buckingham Palace on Monday to protest, before coming down voluntarily.
Mrs Percy, of Hull, who was arrested and later bailed, said she was "amazed" she had been able to unfurl a banner declaring Mr Bush was not welcome in the UK.
The UK Prime Minister Tony Blair urged supporters of Mr Bush to make their voices heard along with those of the protesters.
His spokesman said: "This is precisely the right time for President Bush to
be visiting this country and a view the prime minister believes is widely shared
throughout the country.
"He recognises and accepts there are those who are opposed to the visit, some of them strongly, and that they have the right to make their voice heard.
"But he believes that the majority of people welcome President Bush,
recognise the importance of the relationship with America and note the
commitment he is showing to establishing democracy in Iraq alongside our
diplomats and soldiers."
A Guardian/ICM opinion poll suggests that 43% of people welcome Mr Bush's visit to the UK, compared with 36% who say they would prefer he did not come.
Extra police have been placed at ports and airports and checking people arriving on Eurostar trains from France.
Scotland Yard's Deputy Assistant Commissioner Andy Trotter said there would be 14,000 police shifts worked covering the visit.
The president and his wife arrived in London on Air Force One
"We are on a very high level of alert at the moment," he said. "We obviously have the visit of the president coinciding with that and we've got to make sure that London is kept safe and the visit goes well."
He denied reports that police were considering shutting mobile phone masts during protests against the president's visit.
Buckingham Palace and the Foreign Office have said Mr Wilson's stay at the palace was not a state visit, making Mr Bush's visit a first.
During his visit, Mr Bush will also be protected by hundreds of armed guards from the US.
They will not be granted diplomatic immunity, and will be subject to the British legal system if they shoot anybody, the Home Office has promised.
Mr Bush has shrugged off the prospect of protests, saying he supports free speech and expects the trip to be "fantastic".
After reassurances from the Stop the War Coalition, CND and the Muslim Association of Britain that their mass protest on Thursday will be peaceful, police have agreed they can now march up Whitehall.
Michael Howard, leader of the opposition Conservative party, described the security operation in London as "a price worth paying for freedom".
Charles Kennedy, leader of the UK's third largest party the Lib Dems, urged protesters to "use the opportunity to leave the president in no doubt as to the extent of public concern... about the way in which events tragically have unfolded".
Meanwhile, Azmat Begg, the father of Moazzam, one of the British terror suspects being held at Guantanamo Bay, has called on Mr Bush to release him from
the Cuban military base to face justice on home soil.
As the president and his wife arrive in London on Tuesday, a Stop Bush rally is being held near Euston Station.
And on Wednesday, when the president is due at Buckingham Palace, there will be an "alternative state procession" including a Big Red Peace Bus.
London Mayor Ken Livingstone is holding a peace party in City Hall on Wednesday, attended by many groups opposed to the war in Iraq.