Police searching homes for evidence of a planned terror attack in the Manchester area are reported to be studying photographs found at one location.
Senior officers described claims that shopping centres and a nightclub were among potential targets as speculation.
Millions of people living in the region were urged by police not to let speculation over potential 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-Qaeda plot over the weekend.
Mr Fahy said he and his family would have "no hesitation" in using shopping locations such as Manchester's Trafford Centre and Arndale Centre.
Whitehall sources said the 12 men arrested across the North West on Wednesday were 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 the alleged plotters may have been sizing up "soft targets" such as shopping centres, nightclubs and football grounds like Old Trafford as "speculation".
Security staff at the Trafford Centre said they had not been informed of any threat.
And 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's been some speculation about certain locations, particularly in the North West, concerning this
investigation," he said.
"There is no particular threat against any particular location and certainly not the ones mentioned in the media.
11 Pakistani nationals - of which at least 10 held student visas - and one UK-born British national remain in custody.
Forensics officers will today continue to conduct meticulous searches of the raided addresses in the Cheetham Hill area of Manchester and Liverpool.
It is expected to be a long process which could last up to several days as police examine materials, undertake forensic tests and interview suspects.
Armed officers swooped on at least 14 addresses, including homes, flats, an internet cafe, a car on the M602 and Liverpool's John Moores University, during six hours of frantic activity on Wednesday night.
Among those held were two security guards working at a branch of Homebase in Clitheroe, a student outside the
university library and four men at addresses in the Cheetham Hill area of Manchester.
The raids involving hundreds of officers were brought forward 12 hours because of a blunder by Britain's top
counter-terrorist officer.
Assistant Commissioner Bob Quick resigned after a security breach when he was photographed carrying a secret document containing details of the police operation into 10 Downing Street.
Mr Fahy suggested it was not ideal the operation was brought forward but added officers were used to dealing with "fast-moving situations".
Prime Minister Gordon Brown defended the anti-terror operation but suggested more work must be done to stop lawless areas of Pakistan acting as safe havens for extremists to plot attacks worldwide.
Mr Brown said: "We are dealing with a very big terrorist plot. We have been following it for some time.
"There were a number of people who are suspected of it who have been arrested. That police operation was successful.
"We know that there are links between terrorists in Britain and terrorists in Pakistan. That is an important issue for us to follow through and that's why I will be talking to President Zardari about what Pakistan can do to help us in the future."
Mr Quick's demise came swiftly after photographs taken with zoom lenses in a split second as he got out of his
chauffeur-driven car were circulated on the internet.
His gaffe led to the Defence Press and Broadcasting Advisory Committee, which advises the media on national security matters, issuing a request to news organisations not to print the photograph.
The former Surrey chief constable decided to resign less than 12 hours later after conversations with Metropolitan Police Commissioner Sir Paul Stephenson and Home Secretary Jacqui Smith.
In a statement, Mr Quick said: "I have today offered my resignation in the knowledge that my action could have
compromised a major counter-terrorism operation.
"I deeply regret the disruption caused to colleagues undertaking the operation and remain grateful for the way in which they adapted quickly and professionally to a revised timescale."