The 'secret' documents clutched by Mr Quick were clearly on show
Twelve men have been arrested in the north west of England after Britain's most senior counter-terrorism police officer sparked a security alert.
Assistant Commissioner Bob Quick said he "deeply regretted" revealing a secret document to photographers when he arrived for a briefing at No 10.
The document, clearly marked "secret", carried an outline briefing on an ongoing counter-terrorism operation.
The 12 suspects were later arrested at locations across north-west England.
It is understood raids took place at 10 addresses sooner than planned due to the documents being revealed.
Opposition MPs criticised Mr Quick, with the Liberal Democrats describing him as "accident prone" and the Conservatives condemning his "very alarming" lapse of judgement.
But former Labour Mayor of London Ken Livingstone said it would be wrong for such an experienced officer to resign "for holding a piece of paper the wrong way".
Jacqui Smith praises officers over the terror raids
Home Secretary Jacqui Smith made no comment about the officer's mistake. Instead, she praised police for their professionalism.
"The decision to take such action was an operational matter for the police and the security service," she said, adding that she and the Prime Minister had been kept informed of developments.
Police were shouting at [a suspect] and one of the officers had what looked like a machine gun pointed right into his head
Student Daniel Taylor
Some hours after the Downing Street incident, armed officers from the North West Counter-Terrorism Unit arrested a man outside the main library at Liverpool John Moores University.
Students said they heard police shouting at two suspects as they were advised over the library loudspeaker to stay away from the windows for their own safety.
Journalism student Daniel Taylor said: "I saw a man on the floor. Police were shouting at him and one of the officers had what looked like a machine gun pointed right into his head."
Police wearing blue plastic gloves were searching the man, along with a second man nearby, and both appeared to be "ordinary students", Mr Taylor said.
Ten of those arrested are Pakistan-born nationals on student visas and one is a UK-born British national.
Their precise ages are not known but range between a youth in his mid-to-late teens and a 41-year-old man.
Greater Manchester Police said several hundred officers were involved in the operation, including armed officers during some of the arrests.
Five addresses in the Cheetham Hill area of Manchester, including an internet cafe, are being searched.
Three premises are being searched in Merseyside, plus a guest house in Clitheroe, Lancashire.
Two men are understood to have been arrested at a Homebase DIY store in Clitheroe, where dozens of police officers carried out a raid.
In Cheetham Hill, witnesses described two men being taken from the cafe after police arrived.
Mesu Raza, who lives in the flat above, said: "I saw police arrest two people and put them in a police van. They had handcuffs on, they were Asian men, and the police were armed."
The counter-terrorism officers were assisted by officers from the Merseyside, Greater Manchester and Lancashire police forces.
Detective Chief Superintendent Tony Porter, head of the North West CTU said: "Today's action is part of a continuing investigation and we have acted on intelligence received.
"This phase is still in its very early stages, so the information we can release about it is limited."
Footage of the anti-terror raid at Liverpool John Moores University
Earlier on Wednesday press photographers in Downing Street pictured Mr Quick clutching a white document marked "secret" and containing the names of several senior officers, locations and details about the nature of the overseas threat.
Details of the information revealed cannot be reported.
Mr Quick was attending the meeting in his role as lead for counter terrorism and for the Association of Chief Police Officers (Acpo).
On Wednesday evening he apologised to Met Commissioner Sir Paul Stephenson for the error.
A Scotland Yard spokesman said: "Assistant Commissioner Quick accepts he made a mistake on leaving a sensitive document on open view and deeply regrets it."
Shadow home secretary Chris Grayling said Mr Quick's judgement had been called into question.
"This was highly sensitive information that should not have been carried under an arm in front of a line of photographers," he added.
Liberal Democrat home affairs spokesman Chris Huhne said: "Assistant Commissioner Bob Quick seems to be increasingly accident prone which is potentially dangerous given the serious responsibilities of his role."
Ken Livingstone said: "The idea that we should offer up the person who's at the head of our counter terrorism, with years of experience - a dedicated public servant, for one mistake of holding a piece of paper the wrong way - well, al-Qaeda will be delighted."
Mr Quick, once seen as a "safe pair of hands" by Scotland Yard colleagues, ordered the controversial arrest of senior Tory MP Damian Green over alleged security leaks.
Last December, he accused the Conservatives of involvement in newspaper stories about his wife's hire car business.
It is not the first time secret information has been revealed to journalists who habitually stand outside Downing Street.
Last year housing minister Caroline Flint inadvertently showed off secret Cabinet briefing notes revealing a steep projected fall in UK house prices.
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