People living in Manchester and the North West have been urged not to let speculation over potential terror plot targets affect their Easter plans.
Greater Manchester Chief Constable Peter Fahy said the public should not fear visiting any of the reported targets of a suspected al Qaida plot over the weekend.
Whitehall sources said the 12 men arrested across the North West on Wednesday had been under surveillance by MI5 and police for weeks but the nature or potential target of the plot remained unclear.
One source said: "There was information of sufficient concern that action needed to be taken. Work is ongoing to get to the bottom of it."
The official described reports that the alleged plotters may have been sizing up "soft targets" such as shopping centres, nightclubs and football grounds, like Old Trafford, as "speculation".
Mr Fahy said he and his family would have "no hesitation" in using shopping locations such as Manchester's Trafford Centre and Arndale Centre this weekend.
Security staff at the Trafford Centre said they had not been informed of any threat. A spokesman for the Arndale Centre said there was "no evidence" of any specific targeting of the complex.
Mr Fahy also said police had not uncovered a threat to a particular location, although the investigation was still ongoing.
"Clearly, there has been some speculation about certain locations. There is no particular threat against any particular location and certainly not the ones mentioned in the media."
Eleven Pakistani nationals - of which at least 10 held student visas - and one UK-born British national remain in custody.
