US President Barack Obama's first state visit to Britain has sparked one of the biggest security operation's London has ever seen.
The measures are said to be costing £10m and will see much of the capital enveloped in a ring of steel.
But as on any official trip, the US leader comes with his own vast security team and equipment to keep him safe.
Extra caution is being taken following the killing of Osama bin Laden in a US raid in Pakistan earlier this month.
Mr Obama and his wife Michelle are travelling with an entourage of around 500, including some 200 secret service agents.
As is customary, they have flown to Europe in Air Force One - the presidential Boeing 747 jet.
It doubles as a luxury hotel, meaning they have the chance of a decent night's sleep en route, and also has a kitchen and gym.
The plane has state-of-the-art defences, with armour-plated wings and a special fuselage designed to protect its communication systems.
It is also a fully-equipped command centre, with more than 80 satellite phones on board to make sure the president can always be contacted.
An unnamed military officer carrying the codes needed to launch a nuclear missile attack also accompanies the US leader, just in case.
And a group of six doctors are on stand-by with supplies of Mr Obama's AB blood ready to use in any medical emergency.
Once out of the aircraft, the president travels in his bomb-proof Cadillac nicknamed "The Beast".
It has eight-inch thick armour-plating on the doors, bullet proof glass, a GPS tracker and a panic button.
A motorcade of around 20 cars and a dozen police outriders on motorcycles will surround the vehicle as it moves.
The security operation is so tight that Mr Obama's team even asked Buckingham Palace to chance the glass in his guest suite.
His aides have brought their own portable bomb-proof double glazing to cover the windows.
Some reports also claim the president's men have set up beds in the basement of the Palace and installed their own communication hub.
READ MORE ON THE EUROPEAN TRIP:
:: Barack Obama and his wife Michelle arrive in windy Dublin for a 24-hour stop in Ireland.
:: Moneygall in Ireland has gone Obama crazy as it welcomes its most famous son.