Street patrols and armed police in fortress London

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A massive security operation to guard against a possible terror attack on the anniversary of the 11 September atrocities was launched throughout the United Kingdom today.

A massive security operation to guard against a possible terror attack on the anniversary of the 11 September atrocities was launched throughout the United Kingdom today.

The heightened alerts – at airports, railway stations, London's financial centres and possible American targets – involved thousands of police officers, guards and intelligence agents.

In London, which is considered the most likely target for any attack, extra officers are being deployed on highly visible street patrols and covert surveillance. Armed police checkpoints are being set up at entry and exit points to the City of London financial centre. Canary Wharf in east London, the second most important financial area, is also being given heightened security.

Additional officers will guard mainline railway stations, the London Underground, and vulnerable sites, such as the American embassy and government and diplomatic buildings, and Buckingham Palace. Rapid response units, which include officers trained in dealing with chemical, biological, radiological and nuclear incidents, will be on stand-by.

Extra patrols will be in place at nuclear power stations, military installations and airports.

Businesses connected to American firms, as well as the media and many financial institutions have installed extra security measures, including more guards, security checks and heightened searches in postal rooms for suspicious parcels and letters.

Last month the Financial Services Authority unveiled an emergency nerve centre, which could keep the country's biggest financial institutions moving if a bomb blast closed their normal base.

At Heathrow airport 18 of today's scheduled transatlantic flights have been cancelled by BA, Virgin and American Airlines, while two scheduled flights to New York from Gatwick have been cancelled by the American operator Continental Airlines.

Additional vehicle checks and armed patrols have been planned at airports. But security sources stressed that there was no intelligence to indicate that any organisations, such as al-Qa'ida, were planning to launch an attack in the UK.

Scotland Yard has warned that lone terrorists unconnected to al-Qa'ida could try to exploit the "world stage" offered by today's anniversary.

The head of Scotland Yard's anti-terrorist squad, Assistant Commissioner David Veness, last week urged everyone to be "vigilant" and to report anything unusual.

He said: "We shouldn't just think about the ghastly spectacular attacks that have already taken place, but also the range of possibilities that smaller groups and individuals may use. There may be individuals who think here is an anniversary which is a world stage in terms of publicity and, for their own reasons, they might attack We shouldn't underestimate these individuals."

In the run-up to 11 September about 20 "hardcore" Islamic extremists, living in Britain and suspected of supporting al-Qa'ida, have been arrested on minor charges unrelated to terrorism. Several dozen other suspects are under surveillance by MI5 and Special Branch.

The victims of the 11 September attacks will be remembered in Britain today with the release of more than 3,000 white rose petals from the Whispering Gallery of St Paul's Cathedral.

The petals, representing all those who lost their lives in New York, Washington and Pennsylvania, will rain down during a national service of remembrance attended by bereaved families and guests including the Prince of Wales, Prince Harry and Tony Blair.