Analysis: The terror threat to London

Al-Qaeda's campaign of bombing across the world has struck diverse civilian, military, government and industrial targets.

They include, of course, the twin towers of the World Trade Center and the Pentagon in Washington, and, if al-Qaeda were the perpetrators, the rail terminals of Madrid.

But the terror organisation's campaign also includes smaller targets, such as the nightclub bombings in Bali, and the suicide attack on the private compound of westerners' apartments in Riyadh, Saudi Arabia, in May last year.

This unpredictable pattern of attacks makes it impossible, without intelligence from within the terrorists' camp, to say which buildings, or even which area in London is most likely to be hit by al-Qaeda extremists.

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The response from security chiefs in Britain has been massive, with added security at a host of major targets and frequent public warnings about the threat of attack.

Anti-terror police have arrested more than 500 people, 90 of whom are due to come before courts.

Police have this week asked members of the public to be vigilant in pubs and nightclubs, as well as on trains and the underground.

Since the September 11 attacks security has also been stepped up around Downing Street and the Houses of Parliament, most noticably with the introduction of large concrete blocks around Parliament, also in May last year.

New anti-terror measures have also steadily been put into airports surrounding London. Check-in can take longer in the wake of the New York bombings, with passengers required to remove jackets and even shoes.

Flights have been cancelled and the army has been moved in in the past year, to protect international travellers.

And last weekend it was announced British Transport Police would travel incognito on London Underground as terror marshals on the Tube, equipped to detect Nuclear, Biological and Chemical weapons.

This coincided with the launch of a new anti-terrorism awareness campaign on the Tube, warning passengers to be wary of bags left unattended and other suspicious activity.

Met Commissioner Sir John Stevens even admitted security measures had been stepped up on the Thames river, amid fears it could be used as a route for terrorists to strike the heart of London.

The UK's top policeman left no doubts as to the scale of the threat facing London.

This was echoed by Mayor Ken Livingstone, who said it would be 'foolish' to assume terrorists would not be successful in attacking London at some stage.