Police begin to build 'security bubble' for Bush

The unprecedented security operation to protect George Bush during this week's state visit starts today after the Home Office said Britain was at a heightened state of terror alert, although there was no specific threat.

Up to 4,000 officers will be policing demonstrations by some 60,000 protesters who are demanding the right to march down Whitehall on Thursday, when Mr Bush is to hold talks with Tony Blair in Downing Street.

Extra police will be visible in central London this morning, when protesters will meet police for final negotiations over the route of the march. Defensive barriers at locations on the President's itinerary will start appearing tomorrow evening, when Mr Bush arrives.

All police leave has been cancelled for the duration of his visit, and more than 400 US agents and security advisers will work alongside Scotland Yard's £1m operation. The US Secret Service will be checking all sewers, and will seal manholes and service covers, which will be reopened after the three-day visit.

The American security demands reportedly included draconian measures such as the closure of parts of the London Underground and the use of US Air Force helicopters above the capital. These demands were turned down by the Home Office. But road closures are expected to create a "sterile zone" around the President, who will be shielded by a security cordon.

One issue is whether Whitehall will remain open to a march led by the Stop the War coalition on Thursday, that will culminate with the toppling of a statue of Mr Bush in Trafalgar Square. Mr Bush is to have a working lunch with Mr Blair at No 10 that day, followed by a joint press conference at the Foreign Office. Stop the War Coalition leaders will meet senior Metropolitan Police officers today to hear if they will be allowed access to Whitehall.

Andrew Burgin, of the coalition, said last night that the indications were that Scotland Yard was prepared to be more flexible in return for assurances about stewarding of the march. "We do not want the march confined to a small area north of Trafalgar Square so we have to come to an agreement that means it goes through the heart of the political decision making of the country," he said.

PROTECTING THE PRESIDENT

President George was never going to ride down the Mall in London in an open-topped landau. Security takes priority and it shows in the vehicles he will use: the presidential airplane, Air Force One, his heavily-armoured car and a Black Hawk helicopter on stand-by.

Security on Air Force One is the responsibility of the Secret Service, which sends out teams to test the modified 747's fuel and then seal the tanks. They check runways for debris and can issue orders to shoot if they believe there is a threat to the President. "The security is pretty ferocious on Air Force One," says Peter Schnall, who produced and directed the documentary Air Force One for the Discovery Channel. "It doesn't matter where he flies. The transporting of the President from one place to another is a military operation." The custom-built aircraft, which is protected by anti-missile devices, can fly at 630mph and weighs more than 400 tons. The President is able to have securely encrypted communications with anyone.

The President's limousine, a Lincoln Cadillac DeVille, seats seven and has titanium-ceramic reinforcing able to withstand a rocket-propelled grenade, a 155mm shell, or a 5lb bomb under the car. The windows are splinter-proof glass with polycarbonate laminate up to 2.5 ins thick, resistant against a .50 military calibre bullet. If the tyres are hit, the car can travel at up to 60mph on its steel wheel rims. The radiator is also armoured, and an infrared Night Vision system can be used to "see" people and hot objects in darkness or smoke. It is believed the car also has a radar system.

In the event of a biological attack, the car can be sealed, switching to an onboard air supply. A communications switchboard (linked to five external aerials) is able to connect the President to the White House, Air Force One, or numbers programmed in for his visit, such as Tony Blair or the Queen.

Marine One, a variation on the standard Sikorsky Black Hawk helicopter, is able to cruise at 150mph and travel 580 miles. One of a five-strong squadron, it is "battle-hardened", with duplicate electric and hydraulic systems, and armouring to withstand small arms and explosive fire. The President may travel by this method to Sedgefield, Tony Blair's constituency. If he does fly, his motorcade will travel on the ground. Motorways will be cleared of traffic, and no aircraft will be allowed within 10 miles of the helicopter.

Certainly, it seems a long way away from the visit by John F Kennedy in 1961, when he rode up the Mall in an open-topped Bentley.