LONDON — In the biggest antiterrorist sweep in Britain in nearly two years, police on Monday arrested a dozen men accused of plotting a large-scale terror attack on targets inside the United Kingdom.
The suspects, who ranged in age from 17 to 28, had been under surveillance for weeks and were believed to have links to Pakistan and Bangladesh, security officials said.
The arrests come amid growing concerns in Europe over terrorism after a suicide bombing in Sweden and reported threats of a terror attack on a European city modeled on the deadly shooting rampage in Mumbai, India.
Police swooped in before dawn Monday in coordinated raids on houses in four cities — London, the Welsh city of Cardiff and the English cities of Birmingham and Stoke-on-Trent. The officers were unarmed, suggesting any planned attack was not imminent.
The raid, a joint operation by Britain's domestic spy agency MI5 and police, was the largest since April 2009, when 12 men were detained over an alleged al-Qaida bomb plot in the northern city of Manchester.
Counterterrorism officials declined to give more details of the latest alleged plot, saying only that the men had been under surveillance for several weeks. No details were given as to whether explosives or arms were found, and searches were under way in the homes where the arrests took place.
"The operation is in its early stages so we are unable to go into detail at this time," said John Yates, Britain's senior counterterrorism police officer.
Still, he said Monday's raids, involving a dozen suspects across the United Kingdom, indicated they were planning something big.
"This is a large scale, pre-planned and intelligence-led operation involving several forces," Yates said.
Police have up to 28 days to question the suspects before they must be charged or released.
In Washington, U.S. Secretary of Homeland Security Janet Napolitano said the terror suspects arrested in Britain were not threatening U.S. targets.
Iraqi officials claimed last week that captured insurgents believed the Dec. 11 bombing in Stockholm was part of a series of attacks planned for the Christmas season.
Those claims were rejected by both British and German officials, who insisted there are no specific threats to their countries over the holiday period.