Armed police officers raided homes in several places across the country yesterday, arresting the 13 men - in their 20s and 30s - on suspicion of being involved in the commission, preparation or instigation of acts of terrorism.

While the BBC, quoting eye-witnesses, reported that the arrested men were all of Asian origin, a spokesman of the Metropolitan Police said their nationality "will not be revealed before they are charged with any crime."

Counter-terrorism officials said the arrests were the result of "extensive investigations" which involved Mi5, the British Intelligence Service as well as the police.

"The operation is part of continuing and extensive inquiries by police and the security service into alleged international terrorism," the Metropolitan police said in a statement. The arrests came as Prime Minister Tony Blair came under pressure to elaborate on the nature of the terrorism threat in Britain.

Immigration officers let terrorists into UK

However, Officials distanced the arrests from the terrorism alert in the US, saying it "predated" intelligence coming from Pakistan - a reference to information about potential US targets and addresses found on computer disks in the possession of al-Qaeda suspects there.

But Mi5's involvement in the operation leading to the arrests suggests that it was a result of an intelligence tip-off rather than information gathered routinely by the police. "We are taking it seriously," a senior official said.

While the 13 were taken into custody, other officers carrying search warrants granted under the Terrorism Act 2000 also began searching properties in London, Bushey, Luton and Blackburn.

America's war on terror: Complete Coverage