London - A major anti-terrorism raid in Britain prematurely exposed by a security blunder was linked to a "very big" suspected terrorist plot with possible connections to Pakistan, Prime Minister Gordon Brown said Thursday.

Hours earlier, Scotland Yard's chief anti-terrorism officer, Bob Quick, had resigned after being photographed carrying clearly-visible top secret documents on the raids into a Downing Street briefing with Brown and key cabinet members.

The embarrassing security lapse prompted the raids on 10 localities in the north-west of Britain to be brought forward, resulting in the arrest of 12 men - including 10 Pakistani nationals.

But Brown, clearly aiming to defend Scotland Yard against mounting criticism, said the police were "right to take urgent action" to prevent a "very big attack" and "probing links with Pakistan."

"Let's remember the facts," Brown told the BBC Thursday. "We know there are links between terrorists in Britain and Pakistan and that is an important issue for us to follow through."

Government sources said the men had been under surveillance by the intelligence services for weeks, but the nature and the timing of the plot remained unclear.

"There was information of sufficient concern that action needed to be taken. Work is ongoing to get to the bottom of it," one source told the Press Association.

However, the source dismissed as "speculation" media reports that the alleged plotters may have been sizing up "soft targets" such as nightclubs, football grounds and a shopping centre in the northern city of Manchester.

Police said 10 of the 12 suspects held were Pakistani nationals staying in Britain on a student visa, and one was British.

The raids on homes, an internet cafe, a DIY store, a hotel and a university library, were carried out by heavily-armed anti-terrorist officers in Manchester, Liverpool and the small town of Clitheroe, in the county of Lancashire, Wednesday afternoon.

They were brought forward due to Quick's blunder, for which he apologized Thursday. "I deeply regret the disruption caused to colleagues undertaking the operation and remain grateful for the way in which they adapted quickly and professionally to a revised timescale," Quick said in a statement.

He will be succeeded by John Yates, a Scotland Yard Assistant Commissioner, who attracted criticism from politicians for his thorough investigation into the "cash for peerages" sleaze allegations last year.

Meanwhile, the huge police operation in the north-west continued Thursday, with eyewitnesses confirming that most of the properties targeted were used by Pakistani students.

At Liverpool John Moores University, students said two men, aged in their 20s, were held Wednesday by armed police in dramatic scenes outside the library.

Muslim representatives expressed "concern" at the impact the raids could have on community relations if the arrests were not followed up by charges - as had happened in previous major anti-terrorism raids.

While politicians of all major political parties said Thursday that Quick was right to resign over the security breach, Ken Livingstone, the former Labour mayor of London, said the government and the police had exposed themselves to ridicule.

"Clearly Bob Quick had made a ridiculous error. Whether it justified him being forced out... I think al-Qaeda will be celebrating,", said Livingstone.