Britain’s biggest ever security operation will protect George W Bush when he arrives in London for his controversial state visit tonight.
Scotland Yard is planning an estimated £5m (€7.2m) blitz involving all its armed units and up to 14,000 police officers during the three-day trip.
“The bubble” – a ring of 700 of Mr Bush’s own secret service agents and security advisers – will also surround the President amid fears of terror attacks.
Dozens of specialist firearms squads and other anti terrorism units have also been drafted in.
Sir John Stevens, the Metropolitan Commissioner, said the security would be “unprecedented“.
The President will arrive in Britain this evening on Air Force One, a high-tech deluxe jumbo jet dubbed the “flying White House“.
US officials remain tight lipped about on-board security, but the plane is known to be a military aircraft designed to withstand an air attack.
Its defences include an electronic device to jam enemy radar and flares that can be fired to throw heat-seeking missiles off course.
On September 11, 2001 Air Force One became a mobile bunker for Mr Bush when all ground positions seemed vulnerable to attack.
A US cargo jet is believed to have already delivered the president’s two motorcades to carry him about when he arrives in London.
They are each made up of 20 armoured vehicles, including the President’s bulletproof Cadillac DeVille, vans and Humvees to provide a wall of steel around Mr Bush as he makes his way through the capital.
Some 250 secret service agents, many carrying guns, and 150 national security advisers will also protect Mr Bush during his trip.
Another 50 White House political aides and 200 representatives from other US departments will also accompany him, along with 15 sniffer dogs and their handlers and a personal chef and four cooks.
The Met has said it will deploy every unit of armed officers it has for the operation. It has trebled the number of officers on duty in the capital from 5,000 to 14,000 to provide maximum protection.
Despite the security, one protester was arrested last night after scaling the gates to Buckingham Palace with a banner telling Mr Bush he was not welcome in Britain.
Veteran anti-nuclear campaigner and grandmother Lindis Percy, from Hull, voluntarily climbed down from her perch after about two hours.
The Met has agreed to allow protesters against the war in Iraq to march past Parliament and along Whitehall during Mr Bush’s stay.
Deputy Assistant Commissioner Andy Trotter said that despite the road closures, Mr Bush could “quite easily” come into contact with protesters.
Police were criticised for using “over the top” tactics during demonstrations against the Chinese President Jiang Zemin’s state visit to Britain in 1999.
Officers seized banners from protesters demonstrating over China’s human rights record and in some cases physically restrained activists while the President was passing.