British anti-terror police were scouring homes and interrogating 11 Pakistanis on Friday over what British Prime Minister Gordon Brown called a "major terrorist plot".

Ahead of the Easter holiday weekend, police swooped on several addresses in northwest England, arresting 12 men ? 11 of them Pakistani nationals and most of them students.

The BBC, citing sources, said that during the searches photographs had been found of two major shopping complexes, a nightclub and a square in Manchester, the region's main city.

The Wednesday evening raids had been hastily brought forward by up to 24 hours after the country's top anti-terror chief, Scotland Yard's Assistant Commissioner Bob Quick, was photographed holding documents bearing details of the operation.

The secret document was clearly visible as he arrived at Brown's Downing Street office. Quick resigned over the blunder.

The document included locations of the raids and showed the alleged plot was "AQ-driven," meaning al-Qaeda.

Al-Qaeda operatives located on the Pakistan-Afghanistan border were highlighted as one of the major security threats facing Britain in the government's counter-terrorism strategy published last month.

Brown said he wanted more help from Islamabad on terror, but Pakistan's ambassador said London had to do more to stop the wrong people entering the country.

"We know that there are links between terrorists in Britain and terrorists in Pakistan," Brown said.

"That is an important issue for us to follow through and that's why I will be talking to President (Asif Ali) Zardari about what Pakistan can do to help us in the future.

"We have been investigating a major 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."

But Wajid Shamsul Hasan, Pakistan's high commissioner to London, said there were problems with Britain's student visa system.

"It is at your end you have to do something more. Every day we are raiding people, we are arresting people," he told BBC television.

"If they allow us to make inquiries first, if they ask us to scrutinise those who are seeking visas, we can help them."

Government figures showed 42 292 student visas were issued to Pakistanis between April 2004 and April 2008.

In 2007-2008, the last financial year for which figures are available, 9544 Pakistani nationals were given student visas.

Figures from 2006 showed 98 percent of applications for extension of leave to remain in Britain were granted.

A Home Office spokesman said everyone applying to study in Britain was checked out.

"Every student wishing to study in the UK undergoes scrutiny ? including fingerprint checks against a range of immigration, terrorism and crime watch lists," he said.

"We'll do everything in our power to bar those who are up to no good."

Meanwhile, Greater Manchester police chief Peter Fahy said people should not feel worried about visiting Britain's third city over Easter.

"I would have no hesitation, or any of my family, in using any of those locations that have been mentioned," he said.

The raids targeted properties in Manchester and Liverpool ? including at John Moores University ? and the town of Clitheroe.

Britain has been on high security alert ever since the July 2005 attacks in London, which killed 56 people including four suicide bombers, and failed car bomb attacks in London and Glasgow in June 2007.

The threat remains on its second highest level: severe.