British police arrested 12 men Wednesday in a series of anti-terrorist raids involving hundreds of officers across northwest England.

Greater Manchester Police said the suspects were detained under the Terrorism Act at eight addresses in the cities of Manchester and Liverpool and the surrounding area, about 200 miles northwest of London.

Police would not give any details of the alleged plot. The BBC reported that most of those arrested were Pakistani nationals.

Police said the suspects ranged in age from a youth in his mid-teens to a 41-year-old man.

"Although the operation is ongoing, this phase is still in its very early stages, so the information we can release about it is limited," said Detective Chief Superintendent Tony Porter, head of the Northwest Counterterrorism Unit.

Police would not confirm reports in British the media that the arrests were triggered after a senior officer inadvertently leaked details of a secret anti-terrorism operation.

Britain's top counterterrorism officer, Assistant Commissioner Bob Quick, was photographed entering Prime Minister Gordon Brown's office Wednesday morning holding a dossier on which details of an ongoing intelligence operation were visible.

The Metropolitan Police said Quick had apologized to the chief of the force for the lapse.

Home Secretary Jacqui Smith, the country's top law-and-order official, said she and the prime minister had been kept informed about the raids, but that "the decision to take such action was an operational matter for the police and the Security Service."

Police were searching several properties, including an Internet cafe, in the working-class Cheetham Hill area of Manchester, which has a large South Asian community.

Witnesses said another raid was at Liverpool's John Moores University. Footage recorded on a mobile phone showed armed police standing over a man lying on the floor of a university building.

"Suddenly there was all shouting and commotion outside so I went to the window and saw about eight police officers," said student Craig Ahmed, 24. "One of them was armed and was pointing his gun at two men who were ordered to lie face down on the ground."

The British government currently assesses the country's terror threat level as "severe," the second highest of five possible ratings.

It has been at that level or higher since suicide bombers with ties to Pakistan killed 52 commuters on London's bus and subway system on July 7, 2005.

Since then, two other major attempted attacks in Britain have failed. Two weeks after the July 7 attacks, a group of men attempted a similar strike against the city's transit network — but their bombs failed to detonate. In 2007, attempted car bombings in London's theater district and at Glasgow airport also failed.

Intelligence and security officials say more than a dozen other attempted attacks have been thwarted since 2001. They include an alleged 2006 plot to blow up trans-Atlantic jetliners in mid-air, for which eight men are currently on trial.