Constant vigilence: Agents never leave the president's side

London is one of only 15 cities outside the United States to have a dedicated US Secret Service office.

So when the service's director, W Ralph Basham, began sending agents ahead of President George Bush's state visit, he already knew what to expect.

What is without doubt is that the security surrounding the visit will be unprecedented.

And as the president is a man who travels with his own bullet and bomb proof limousine, a mobile operating theatre and armoured helicopter, it will also be something of a spectacle.

For this state visit, President Bush's security is basically split into two parts - the US's own security arrangements and the British contribution.

US security plans

The US Secret Service (USSS) naturally says very little about its work but its primary responsibility is to protect the resident of 1600 Pennsylvania Avenue, Washington.

US SECRET SERVICE Primary role to protect president Hand-picked officers Armed and authorised to fire 250 expected in London Liaising with Metropolitan Police and MI5

It classifies any major overseas visit as a "national special security event".

This triggers a classified presidential security order authorising agents to work with the hosts to create a safe environment for the "permanent protectees" - the president and First Lady.

Weeks if not months ahead of this visit, the USSS advance team arrived in London to examine the capital's security.

Their report covered everything from how to defend motorcade routes, rooftop sightlines for snipers, potential threats from the sewer or Underground system and even how to "deny" airspace to aerial attackers.

The advanced party then laid down plans for worst case scenarios, such as where to place mobile medical teams, how and to where they would evacuate the president, and emergency landing sites for the presidential jet, Air Force One, or the helicopter, Marine One.

Taken together, these surveys have dictated the manpower and equipment provided by both Washington and London.

It is thought the US is bringing 250 armed agents, along with the physical security of the motorcade itself, creating what is known as the impregnable security "bubble".

These preparations form the "Operational Security Plan" which is being managed from the London USSS field headquarters.

In short, the USSS says its strategy is one of "prevention and deterrence" - but it warns it is "prepared to respond tactically to a threat if the situation dictates."

It is at this point that the relationship with the Metropolitan Police becomes critical.

Role of the police

The organisation with the most to do in the UK with the president's visit has been London's Metropolitan Police.

MOTORCADE SECURITY Armoued vehicles can withstand direct attack Close and medium distance air support Heavy arms and terrorism counter-measures available Mobile medical unit Secure and constant communication lines with Washington

It believes that it has the world's most experienced team in policing the two issues that most concern the Americans - the threat of a terrorist attack and the threat of violent protests.

One of the capital's most senior officers, deputy assistant commissioner Andy Trotter, has been negotiating and planning with Washington.

And throughout President Bush's visit, control of up to 5,000 police officers on the ground will pass to the operation's "Gold Commander", Michael Messenger.

According to Mr Trotter, Mr Messenger is one of the world's most experienced officers in managing city-wide public order and security operations.

He was involved in the delicate and controversial arrangements for the state visit of Chinese President Jiang Zemin in 1999.

This time, the Met says there will be no "exclusion zones", rather there will be rolling road blocks.

What this usually entails is advance police units temporarily closing off roads in the minutes before a secure cavalcade passes.

Members of the public are not usually moved off the route during these operations which is why Mr Trotter says the president could "quite easily" see demonstrators.

Armed agents

Another critical issue is the relationship between the police officers and the presidential armed agents.

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These agents have been issued undisclosed "rules of engagement".

But given they are hand picked to protect the president at all costs, this has raised fears they would be more likely to resort to an armed response in an emergency than the police.

The home secretary has insisted that although agents may carry arms, they would not be immune from prosecution if they used them.

And for its part, the Met has dismissed suggestions it is expecting violent protests.

There will be armed police in central London and riot-trained reserve officers from the Met's territorial support group will be stationed out of sight but near to all major points.

But the police insist there will be no flight exclusion zones, London Underground station closures or emptying of office blocks.

Individual police officers are being authorised to use extensive stop, search and arrest powers under the Terrorism Act - though in a "refined" way after their controversial use at a recent arms fair protest.

All of this, says the Met, means the president gets his security in an atmosphere of heightened alert, while it also facilitates the right to peaceful protest.