Scotland Yard anti-terrorist officers are still questioning 12 men who were arrested in a series of raids across England yesterday.
Details of the operation were only revealed late last night, with the suspects due to be taken to a central London police station.
The men, all in their 20s and 30s, were detained under Britain's Terrorism Act during raids in Hertfordshire, north west London, Luton, and Blackburn in Lancashire.
Scotland Yard declined to say whether the operation was linked to the seizure of computer files from al-Qaeda suspects captured in Pakistan.
A 13th man, also detained yesterday in Willesden, north London, had since been released with no further action.
