Police said the men, in their 20s and 30s, were suspected of involvement in the commission, preparation or instigation of acts of terrorism.

The men are reported all to be of Asian origin but it is not yet known how many are British citizens.

In a statement on Tuesday, the Metropolitan Police said the raids were part of a "pre-planned, ongoing, intelligence-led operation".

The statement said: "Today's operation is part of continuing and extensive inquiries by police and the security service into alleged international terrorism."

A police spokesman said the suspects would be brought to a central London police station for questioning. Officers are continuing to search "residential premises" in London, Luton, Blackburn and Bushey, Hertfordshire.

The searches are expected to take some time to complete.

The raids come as Prime Minister Tony Blair faced pressure to spell out the level of the terrorism threat in the UK following a heightened alert in parts of the US.

The Home Office has said no specific threat has been uncovered, saying: "We are maintaining a state of heightened readiness in the UK."

According to Home Office figures, by May this year only 14 of the 562 people arrested under the Terrorism Act since 11 September 2001 had been convicted of charges under it.

Massoud Shadjareh, chairman of the Islamic Human Rights Commission, said the group was "extremely concerned" about the timing of the raids just after the alert status was raised in the US.

The raids also come the day before the parliamentary human rights group publishes a report "which is critical of the Terrorism Act" and the treatment of those held under it, he said.

"The timing is very worrying and it is extremely annoying, especially when it actually affects the lives of ordinary people and they are suffering," said Mr Shadjareh.

"It is increasing Islamophobia."

Though a Pakistani intelligence official claimed to AFP in Islamabad that a tip-off from Pakistani investigators led to Britain's arrest of a senior Al-Qaeda operative, BBC home affairs correspondent Daniel Sandford said planning for the UK operation appeared to have been ongoing before arrests in Pakistan triggered the US alert, making a direct link unlikely.