British police were questioning 12 men on Wednesday, arrested in major anti-terror raids across the country, as lawmakers and community leaders said Muslims were being targeted too often.
Few details have emerged about Tuesday's operation but newspapers speculated that police had targeted suspected al Qaeda cells.
Security services say foreign al Qaeda operatives are active in the UK and that there is also a threat from Britons sympathetic to the ideology of Osama bin Laden.
Police said only that the men, in their 20s and 30s, were suspected of "being involved in the commission, preparation or instigation of acts of terrorism" and were arrested as part of a pre-planned, intelligence-led operation.
"Nothing hazardous" such as chemicals or bomb-making equipment had been found, a police source told Reuters, adding all the suspects were believed to be Asian.
A 13th man arrested in the raid was freed without charge.
"These are important arrests and the decision to take action was made after careful consideration," the source said.
They came a day after the United States warned that financial centres in New York and Washington might be attacked by al Qaeda bombers.
That warning followed Pakistan's seizure of a suspected al Qaeda computer expert along with computers and documents which also indicated threats against unspecified targets in the UK, according to British newspapers.
Police declined to say whether there was a link between intelligence from Pakistan and the UK raids, which took place in north-west London and in the southern English counties of Hertfordshire and Bedfordshire as well as the north-western area of Lancashire.
However the police source told Reuters planning for the operation had been under way before the Pakistan arrests.