Britain's top anti-terrorism chief has been forced to resign after he inadvertently released details of an operation that then had to be initiated earlier than expected. In that operation, a dozen men in northwest England have been arrested and are being questioned over alleged terrorism offenses.

When Scotland Yard's Assistant Commissioner Bob Quick arrived at 10 Downing Street to brief the prime minister on security plans, he was photographed carrying documents in clear view that disclosed the operation in detail.

Because of this, the decision was made to move up the police action to apprehend suspects earlier than had been planned.

Students shocked as police arrest men at University

In that unusual daylight operation, men ranging in age from teens to early 40s were rounded up in Liverpool and in two areas outside of Manchester, one to the north of the city and one to the south.

The police action at Liverpool's John Moores University was typical. Students in one of the libraries were shocked as the events unfolded.

"The first thing we knew was an announcement on the Tannoy that suggested that everyone clear their tables and get into the center of the room and keep well away from the windows," an eyewitness said. "Then next thing, everyone was looking around to see what is going on. We looked straight out the window and we saw the police with the guns and the two men with their arms behind their back."

One of the suspects was arrested. The other man was released.

Home secretary praises officers involved in swoop

Despite having been forced to bring the major operation forward, Britain's top law-and-order official, Home Secretary Jacqui Smith praised the hundreds of officers involved in the major swoop.

"While the action was an operational decision for the police and the security services, the prime minister and I were kept fully up to date with developments," Smith said. "We face a serious threat from terrorism in this country and I think we are well served by the excellent work of our police and security services who help to keep us safe."

Pakistanis among those arrested

Of those in custody and being questioned, 10 are Pakistani nationals in Britain on student visas.

They are being held under Britain's anti-terrorism laws.

Police have not provided any information about why they conducted the operation other than to say they acted on intelligence received.

The government currently puts the country's perceived terror threat level at "severe." That is second highest of five ratings.