(CNN) -- British police Wednesday arrested 12 people in a counterterrorism operation, and locations were being searched, authorities said.
A Scotland Yard official's papers show details of the raid, which have been obscured in this photo.
Arrests were carried out in a series of raids in northwest England, police said. Participating agencies included Merseyside Police, Greater Manchester Police and the Lancashire Constabulary, according to a statement from Greater Manchester Police.
The men arrested were involved in a "very serious" plot closely associated with al Qaeda and escaped al Qaeda operative Rashid Rauf, whom British intelligence have linked to the 2006 plot to blow up trans-Atlantic airliners, according to a security source with knowledge of the investigation.
The new plot was not believed to be targeting national infrastructure, such as rail lines, airports or utilities, nor was it clear if the plot was to involved bombs or an assault involving gunmen, the source said.
Details, the source said, were speculative at this point in the investigation.
The source also said authorities don't believe the targets would have been in the north of England, where the arrests took place, and that at least some of those arrested were Pakistanis in the United Kingdom on student visas.
Several hundred officers were involved in the raids, according to a later Greater Manchester Police statement. The men arrested range in age from a youth in his mid- to late teens to a 41-year-old, the statement said. No further information was available, police said.
"Today's action is part of an ongoing investigation and we have acted on intelligence received," said Steve Ashley, chief superintendent of Merseyside police.
"We understand that this kind of police activity can cause concerns to people living in nearby communities. The extra patrols, cordons and measures we have in place have been implemented to make sure we are doing everything we can to reassure the public and maintain public safety."
Home Secretary Jacqui Smith, in a written statement, congratulated police for the "successful anti-terrorism operation which has resulted in 12 arrests at a number of locations."
She said the actions were an operational decision by police and Security Services, but she and British Prime Minister Gordon Brown were kept apprised.
Police rushed to make the arrests after press photographers on Tuesday snapped images of the U.K.'s chief terrorism officer as he got out of a car at the prime minister's residence, according to the security source.
Metropolitan Police Assistant Commissioner Bob Quick was carrying a document containing the names of those to be arrested, the source said, and the photographers were able to easily read the names when they enlarged the photographs.
But, the source said, the arrests would probably have taken place Thursday, and preparations for the arrests were the reason for Quick's visit to see Brown.
"Tonight, the focus is the ongoing operation. That's my priority," Smith said.
Scotland Yard issued a statement regarding the incident.
"Quick accepts he made a mistake on leaving a sensitive document on open view and deeply regrets it. He has apologized to the commissioner and the colleagues."
CNN's Andrew Carey and Laura Perez-Maestro contributed to this report.