LONDON — Britain's top counter-terrorist police officer resigned Thursday after he was photographed carrying clearly visible secret documents about an operation against an alleged al-Qaida plot by Pakistani nationals to launch an attack in Britain.
Assistant Commissioner Bob Quick's blunder forced police to scramble to round up the suspects sooner than planned. Twelve men were arrested late Wednesday in raids across northwest England.
Prime Minister Gordon Brown said the raids had disrupted "a very big terrorist plot."
"We have been following it for some time," Brown told reporters.
Greater Manchester Police Chief Constable Peter Fahy said police had not identified a threat to any particular target. But he said the raids had been triggered because police thought public safety was at risk.
"We perceived a threat was there and we had to take action," he said.
"What happened essentially meant we have brought the matter forward but it would have happened in the next 24 hours in any event," he said.
Police said 11 of the men arrested were Pakistanis, most on student visas, and the twelfth was British. The suspects ranged in age from the teens to a 41-year-old man.
Several past terrorist plots in Britain had links to Pakistan, including the July 7, 2005 London transit attacks by four suicide bombers that killed 52 commuters.
"We know that there are links between terrorists in Britain and terrorists in Pakistan," said Brown. He said he would be asking Pakistani Prime Minister Asif Ali Zardari to take tougher action.
The British government currently assesses the country's terror threat level as "severe," the second-highest of five possible ratings. It means the government considers an attack likely.
Quick, the Metropolitan Police anti-terror chief, was photographed Wednesday clutching confidential documents as he arrived for a meeting with Brown at 10 Downing St. The document on top showed details of an anti-terror operation code-named Operation Pathway.
When officials became aware that clearly readable photographs of the document _ which listed names of senior officers and plans for a series of raids _ were in circulation, they changed their timetable for action.
News organizations were warned by a joint government-media body Wednesday that "publication or broadcast of any details of this photograph would seriously damage national security."
Hundreds of officers swooped on eight addresses across northwest England Wednesday evening _ a contrast to the dawn raids usually favored by police.
Greater Manchester Police said the suspects were detained under anti-terrorism laws in the cities of Manchester and Liverpool and the surrounding area, about 200 miles (300 kilometers) northwest of London.
The raids targeted homes, an Internet cafe, Liverpool's John Moores University and a car driving along a highway.
Once the raids were over, Quick swiftly stepped down.
"I have today offered my resignation in the knowledge that my action could have compromised a major counterterrorism operation," he said in a statement Thursday.
It's not the first time officials calling on the prime minister have been caught out by photographers standing in Downing Street with powerful telephoto lenses.
In May, two government ministers were snapped carrying sensitive materials that could be seen. Caroline Flint, who was then minister for housing, was holding a document forecasting a 10 percent drop in British house prices _ a bigger fall than the government was then predicting.
Hazel Blears, the communities minister, was photographed with an e-mail on the possible participation of the prime minister in a TV reality program to be called "Junior PM."
"I'm not the first person to have been caught out in this way and probably won't be the last," Flint said.
Quick's blunder was the first to raise a security issue and force a resignation.
Law enforcement officials said they were not aware of any instances in which a readable image of Quick's document was published before Wednesday's raids.
After the raids took place, television news reports showed images of Quick holding them. Newspapers also carried the photos, with The Daily Telegraph and the Evening Standard showing close-up images that were clearly readable.
Quick's replacement in the top terror job, Assistant Commissioner John Yates, has been involved in several prominent cases, including an investigation into whether knighthoods and other honors were being given in exchange for Labour Party donations.
At the end of the investigation in 2007, which included police questioning of then-Prime Minister Tony Blair, prosecutors did not charge anyone.
Associated Press Writers Robert Barr and Jill Lawless in London contributed to this report.