LONDON - In the biggest anti-terrorist sweep in nearly two years, British police yesterday netted a dozen men accused of plotting a large-scale terror attack on targets inside the United Kingdom.

The raid was thought to be linked to UK targets only and not part of a larger threat to other European countries, security officials said.

Police who swooped in on the men's houses early in the morning were unarmed, suggesting any planned attack was not imminent and the suspects were not believed armed. Police were heavily criticized in 2006 when they shot an unarmed suspect in a similar counterterrorism raid. Only a fraction of Britain's police officers are armed.

The men were arrested in London, the Welsh city of Cardiff and the English cities of Birmingham and Stoke-on-Trent and the raid was the largest since April 2009, when 12 men were detained over an alleged al Qaeda bomb plot in the northern city of Manchester.

Counterterrorism officials declined to give more details of the alleged plot, saying only that the men had been under surveillance for several weeks - an indication that the plot could have been in its planning stages. No details were given as to whether explosives or arms were found.

"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.

Officers said the men range in age from 17 to 28. Police have up to 28 days to question them before they must be charged or released.

The men are thought to be British nationals but with links to Bangladesh and Pakistan, according to a counterterrorism official who spoke on condition of anonymity because he was not authorized to speak to the media. Britain is home to large Pakistani and Bangladeshi communities.