Suspects were being questioned today after one of the biggest anti-terror operations since the 7 July attacks exploited lax student visa regulations to enter the UK from Pakistan, according to Whitehall sources.

As police continued to search ten addresses in Liverpool, Manchester and Clitheroe, Lancashire, after the raids on Wednesday, the Home Office said student visa checks had been tightened in the last fortnight because of widespread abuses of the system.

It was reported today that in one of the raids police found photographs of four popular Manchester locations. Last night counterterrorist sources said they had uncovered no definite targets for an alleged plot, and described reports citing a shopping centre and nightclub in Manchester as targets as "wide of the mark".

But the BBC said that the police were studying the photographs, found in one raid, of the Arndale and Trafford Centre shopping complexes, Birdcage nightclub and St Ann's Square.

There are concerns inside government and the security services that the 11 Pakistani nationals being held in the north of England could have gained entry on student visas in order to form a sleeper cell. Gordon Brown talked of the police having foiled a "very big terrorist plot".

The operation which led to the arrest of the men, along with one Briton who is said to have roots in the same tribal area, was rushed forward after the country's top anti-terror officer carried papers under his arm detailing the raids as he walked into 10 Downing Street in full view of photographers.

Apologising for the blunder, Bob Quick, the Met's head of specialist operations, resigned from his post yesterday. His departure reignited tensions over the running of the force after London mayor Boris Johnson broke the news of the resignation on BBC Radio 4, angering the home secretary, Jacqui Smith, and Scotland Yard.

Despite the gaffe, security sources said the operation was a success, describing it as an unprecedented joint action by MI5 and MI6 in light of intelligence.

One Whitehall source said the police feared attacks were planned for the Easter weekend.

They said the plot indicated al-Qaida was adopting new tactics to send "clean skins" – people not known to security services or the police – in from abroad, rather than using British-born terrorists to carry out attacks.

The prime minister said: "We are dealing with a very big terrorist plot … 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."

Asked if this would become another high-profile raid ending with no one charged with terror offences, the chief constable of Greater Manchester, Peter Fahy, said: "There will always be situations where … either we can't achieve the evidential threshold or as a result of the investigation we find that the threat was not how it appeared to us at the time."

Fahy urged people in the region not to let speculation over potential targets affect their Easter plans. He said he and his family would have "no hesitation" in using shopping locations such as Manchester's Trafford Centre and Arndale Centre.

Greater Manchester police, which was co-ordinating the anti-terror arrests, said several properties were being searched. They refused to comment on what had been found, but if the threat was as great as sources suggested, officers would have been looking for bomb- making equipment and seizing computers to search for evidence of a cell in operation.

Police were busy at five addresses in Manchester, and on the M602 motorway.

Forensic teams removed several crates of items from a four-star B&B in Clitheroe, Brooklyn House, where the two men who were arrested at the town's new Homebase store had been staying since Monday.

Searches were also being carried out in Highgate Street, Liverpool, where five men were arrested, one from Liverpool John Moores University campus.

Muhammad Adil, a student swept up in the raids at John Moores University but released after a couple of hours, said one friend was still being held. Adil, from Peshawar, Pakistan, said his friend was an accountancy student from Karachi. Adil had been studying in the UK for two years and met his friend at his part-time job as a security guard. "They asked me if I knew why I was being arrested – as suspect of terrorism, I was laughing at that. I've been studying for the last two years," he said.

A Home Office spokesman said that student visa regulations had been tightened so that all would-be students had to have their fingerprints checked against terror and police lists and had to be sponsored by a legitimate college or university in the UK.

Only last month, immigration minister Phil Woolas described how "abuse of the student visa has been the biggest abuse of the system, the major loophole in Britain's border controls".

Wajid Shamsul Hasan, Pakistan's high commissioner to the United Kingdom, said his country could help carry out visa checks, but was not allowed to. He said: "It is at your end you have to do something more. Every day we are arresting suspects wherever we find them."

Asked by BBC's Newsnight if the there was a problem with the British system for student visas, he replied: "Yes. If they allow us to make inquiries first, if they ask us to scrutinise those people who are seeking visas we can help them."

Last year, 9,300 students entered the UK from Pakistan. British colleges now have to register with the UK Border Agency, which last month revealed it had turned down 460 of the 2,100 colleges which had applied for licences to admit international students, because they were bogus establishments sponsoring students as part of an immigration scam.

The fact that Wednesday's arrests were made so fast after Quick inadvertently disclosed secret operational details, indicated that MI5 teams were watching their targets closely. "Things were moving anyway, executive action [arrests] wasn't far off. Quick hastened it," said one official.

Two Whitehall sources indicated the surveillance operation began a fortnight ago when a foreign intelligence agency passed information to British security services. Three days ago, the sources said, further intelligence indicated that any attack the cell was planning was "imminent", and the decision was made to arrest the alleged network. However, that sequence of events was disputed by other sources.