LONDON -- In the biggest anti-terrorist sweep in nearly two years, British police on Monday 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-Qaida 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.
A British security official, speaking on condition of anonymity because of the sensitivity of his work, said the arrests were not thought to be part of any planned holiday season attack. Iraqi officials had claimed last week that captured insurgents believed a recent suicide bombing in Stockholm was part of a series of attacks during 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 festive period.
In October, the U.S. State Department advised American citizens living or traveling in Europe to be wary amid reports that terrorists were planning attacks on a European city.
Some of the details of a Mumbai-style shooting spree plot directed at cities in Britain, France or Germany came from Ahmed Siddiqui, a German citizen of Afghan descent who was captured by U.S. troops in Afghanistan in July. More than 170 people were killed during an attack in the Indian city of Mumbai in 2008.
Another government official on Monday downplayed reports that the latest raids in Britain were part of larger terror concerns across Europe.
"Although serious, we believe this raid may have been a one-off and not necessarily related to larger European terror plot concerns," said the official who spoke on condition of anonymity because he was not authorized to speak to the media.
A third government official, who also spoke to the AP on condition of anonymity, said other plots being monitored within Britain had threads that linked back to that Europe-wide plot reported in October but that there were no new credible or specific "Christmas" terror plots.
Europe has been the target of numerous terror plots by Islamist militants. The deadliest was the 2004 Madrid train bombings, when shrapnel-filled bombs exploded, killing 191 people and wounding about 1,800. A year later, suicide bombers killed 52 rush-hour commuters in London aboard three subway trains and a bus.
In 2006, U.S. and British intelligence officials thwarted one of the largest plots yet, a plan to explode nearly a dozen trans-Atlantic airliners.
British police said Monday's raids and the number of arrests across the U.K. showed that the suspects were planning something big.
"This is a large scale, pre-planned and intelligence-led operation involving several forces," Yates said.
Associated Press writer David Stringer contributed to this report from London.