Some 14,000 police officers will front a massive security operation to protect President Bush when he begins his state visit to the UK tonight.Frank Millar, London Editor, reports.
RAF fighters will enforce a no-fly zone over London, while on the ground US secret service agents will join special forces and every branch of the British security services in an unprecedented effort to make the capital safe.
This is amid renewed intelligence about a possible terrorist threat.
The deputy assistant commissioner of the Metropolitan Police, Mr Andy Trotter, said the terror threat was not directly linked to the Bush visit. He spoke of "a generalised threat level to the UK" which was causing police "some concern".
Tensions rose further last night when the radical Islamist organisation al-Muhajiroun warned Muslims to stay away from a Stop the War demonstration on Thursday because of what it described as the "real and serious risk" of a terrorist attack.
The police last night settled on the route for Thursday's demonstration, which organisers hope will bring 100,000 people onto the streets as Mr Bush holds talks with British Prime Minister, Mr Tony Blair.
However, with early estimates of the capital's policing bill alone being put at over £5 million, questions were being asked about the failure of the authorities to prevent an early demonstrator from chaining herself to the gates of Buckingham Palace for two hours last night.
With drains already checked for bombs, airport-style metal detectors will be used in Whitehall and elsewhere, while rolling concrete barricades will be used to create temporary sterile zones around the locations for the key events of the presidential visit.
The President's personal security staff flew into Mr Blair's Sedgefield constituency yesterday, ahead of Mr Bush's visit there after he takes his leave of Queen Elizabeth.