Thirteen suspected terrorists arrested in a series of raids across Britain yesterday were being questioned today.
Police said the men, all in their 20s and 30s, were seized on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
Scotland Yard, which led the operation, refused to discuss whether the raids were linked to the seizure of computer files from al-Qaeda suspects captured in Pakistan.
The suspects were taken to Paddington Green police station in central London to be interviewed by officers from the Metropolitan Police's Anti-Terrorist Branch.
Witnesses today described how armed officers swooped in a series of raids in north west London, Bushey in Hertfordshire, Luton in Bedfordshire and Blackburn, Lancashire.
In a statement last night, Scotland Yard said the raids were part of a "pre-planned, on-going intelligence-led operation".
The arrests came as the British government came under pressure to spell out the level of the terrorism threat in Britain, as a massive security operation continued in US cities.
America is on heightened alert following the discovery of documents in Pakistan which apparently mention potential targets for attack both in the US and Britain.
However, questions have been raised over the scale of the response ordered by Washington on Sunday, after it was revealed that much of the information seized may have been gathered several years ago.