Security in the capital was tightened on Wednesday, following warnings to Scotland Yard that terrorists connected to the al Qaeda are planning further attacks.

Police chiefs called on the public to maintain a state of "alert but not alarm".

"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," said assistant commissioner of the anti-terrorism squad, David Veness.

"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."

The City of London was under particular scrutiny, along with the London offices of American corporations.

Last month, a "nerve centre" was established at a secret location by the Financial Services Authority to ensure trading continues in the event of terrorist activity.

Heightened protection was also awarded to notable buildings in the Westminster area, including Downing Street and government departments.

Royal buildings were also expected to feature on the list.

Measures were also taken at London's airports; 18 flights to the US from Heathrow were cancelled by BA, Virgin and American Airlines, while two were cancelled from Gatwick.

Both airports observed the minute's silence at 1.46pm.