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When President Barack Obama touched down on British soil, a massive security operation, months in the planning, was under way. Who, and what, is involved?

The Queen has hosted hundreds of foreign dignitaries but none is quite like the president of the United States.

Unlike other state visitors, the president always supplies his own transport, which means he won't be seen in a royal carriage but will be hidden inside the motorised fortress above, known as "The Beast".

Three weeks after ordering the raid that removed Osama Bin Laden, the security operation surrounding President Obama's three-day visit is one of the tightest ever mounted in the UK.

He will bring with him political aides, inscrutable secret service agents and hi-tech vehicles. Some reports suggest this entourage will top 500.

A lot of the advance work would be carried out by US Secret Service agents resident at the American Embassy in London, says Fred Burton, a former Diplomatic Security Services agent. But the Presidential Protection Division typically sends its own agents too.

Working closely with British officials, the US team will have first surveyed the sites on the visit and then identified locations of particular vulnerability. This could mean several preparatory trips to the UK.

The day before the president arrives, there's a sweep of the venues for bugs. The route due to be taken is assessed, which involves checking any fixed object along the way - dustbins, cars, sewers, fire hydrants, lamp posts, letter boxes and even bikes.

Sniffer dogs hunt for explosives, while officials check for evidence of chemical or biological threats.

Transport for London will be informed if Tube trains need to be stopped for a few minutes underneath the street, says Mr Burton, and manholes may also be locked down.

About 30 days ahead, the US starts background checks on everyone the president is likely to come into contact with.

"They are checking for any miscreants or anyone in the intelligence database, looking for a sleeper or an assassin hiding in the midst.

"This could be thousands of people. It's any public location he may go to, like a hotel - but low-level criminal activity would not usually be a problem."

The intelligence effort is extensive, even involving CIA stations around the world looking for any information related to the UK.

The president and his wife Michelle arrived at Stansted Airport on Air Force One, a modified Boeing 747, on Monday night, a day earlier than planned due to the drifting ash cloud from Iceland.

The Obamas had been due to travel by Marine One helicopter (above) to central London and land in the garden of Buckingham Palace, but instead were taken by car to the US ambassador's house.

The VH-3D helicopters are flown by US Marine Corps pilots and always fly in groups of identical helicopters, with as many as four decoys.

They are transported overseas with all the presidential vehicles in a miltary transport carrier, thought to be a C-5 Galaxy.

These choppers are capable of firing flares to divert heat-seeking missiles, have twin engines and are equipped to deal with surface-to-air and air-to-air attacks. The UK is responsible for clearing the air space.

The size of the motorcade will vary according to the risks of the visit, but there could be dozens of vehicles, including decoys.

British police cars and motorbikes lead the way, alongside US agents, to secure the route, followed by secret service vehicles, support cars and emergency medical vehicles.

A three to four-vehicle group will hold the president and senior officials, and at its centre is the armoured limousine. Nicknamed "The Beast", this modified Cadillac has been described as the safest vehicle on the planet, assembled at a cost of $300,000 (£186,000).

It has 20cm (eight inch) armour-plated doors and tyres that still work after being shot through.

But no anti-ramp technology, it appears - the limo came a cropper in Dublin when it became beached leaving the US Embassy. A Secret Service spokesman later said "it was a spare limo".

There is also a counter-assault car in the convoy, which has the job of attacking any hostile threat.

Officials would have identified vulnerable spots along the route, says Mr Burton, who is vice-president of global intelligence firm Stratfor.

"These are called choke points, and they have to try and take them away from the bad guys by physically taking control of them and occupying them."

A commonly used phrase is that where the president goes, America travels with him.

A team of chefs prepare all his food - apart from that served at the official functions - both for convenience and security.

And a personal medical team accompanies the president, equipped with a supply of his blood type AB. The UK will also have a trauma team and ambulance standing by.

It's a remarkable choreography of logistics, says Mr Burton, and the strict control imposed by the US team usually ruffles a few feathers of the host.