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updated 12/21/2010 3:48:06 PM ET 2010-12-21T20:48:06

A large-scale terror attack was aimed at British landmarks and public spaces, security officials said Tuesday as more details emerged and police searched the homes of 12 British suspects being held for questioning.

The men β€” whose ages range from 17 to 28 β€” were arrested Monday in the largest counterterrorism raid in nearly two years. At least five were of Bangladeshi origin.

Lord Carlile, the government's independent watchdog for terror legislation, said Tuesday that the alleged plot appeared significant and involved several British cities, but he did not identify the targets. Police have up to 28 days to either charge the men or release them.

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Possible targets that were scouted include the Houses of Parliament in London and shopping areas around the U.K., according to a security official who spoke to The Associated Press on condition of anonymity because the investigation was ongoing.

The plot, however, was unconnected to a larger European plot uncovered in the fall involving a Mumbai-style shooting spree in cities across Britain, France or Germany, the official said. Monday's arrests were also unrelated to last week's suicide bombing in Sweden and the plot did not appear to be timed for the holidays, he said.

Scotland Yard Assistant Commissioner John Yates said police searches of the men's homes could take days, and officers were seen Tuesday removing computers from their homes. The men were arrested in the cities of London, Cardiff, Stoke-on-Trent and Birmingham.

European officials typically step up security around the holidays, but this year especially after a Nigerian man with explosives taped to his underwear tried to blow up a plane on Christmas Day last year as it approached Detroit.

Umar Farouk Abdulmutallab, who had studied in London, had boarded the Northwest Airlines flight to Detroit from Amsterdam. The 24-year-old is accused of trying to blow up the flight. A judge in a federal court in Detroit has entered a not-guilty plea on Abdulmutallab's behalf.

There have been growing concerns in Europe about holiday season attacks following a suicide bombing in Sweden and security services' fears of an assault on a European city modeled on the deadly shooting spree in Mumbai, India.

Iraqi officials claimed last week that captured insurgents believed the Stockholm bombing was part of a series of Christmas season attacks.

Britain's terror alert has remained unchanged at "severe" while Germany upped its terror alert Nov. 17 in response to an increased threat from Islamic extremists. Berlin's Interior Minister Ehrhart Koerting said recently the threat has eased but said it was necessary to remain vigilant.

Italian officials were on Tuesday investigating whether an explosive device found in a subway car in Rome was terror-related.

The device, full of powder and wires, was found Tuesday inside a train at the Rebibbia station, on the outskirts of Rome. The train was at the end of the line and empty when the package was found.

Bomb-disposal experts checked the powder and concluded the device could not have exploded, Rome Mayor Gianni Alemanno said.

French Interior Minister Brice Hortefeux, in a newspaper interview Sunday, said for the holidays he's ordered plainclothes police patrols in "the most sensitive sites" in France. For New Year's Eve, Hortefeux said he has ordered 71 extra police units to be on call β€” 6,000 more police than normal.

Story: Italy police: Suspected explosive device had no detonator

"All the means have been mobilized so that everyone can go about their holiday season in total peace of mind," he was quoted Sunday in an interview with Le Parisien newspaper.

In October, the United States warned its citizens living or traveling in Europe to be wary after details emerged of a larger European terror threat.

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Monday's 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. All were released without charge and ordered to leave Britain.

One, Abid Naseer, was later re-arrested at the request of the U.S., which says he was involved a plot to bomb the New York City subway. He is in jail awaiting extradition.

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.

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.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Video: In U.K, 12 held in suspected terror plot

  1. Closed captioning of: In U.K, 12 held in suspected terror plot

    >> and we are following developments involving a massive operation aimed at stopping a suspected terrorist plot in britain that has ties to al qaeda . 12 men were arrested in a series of predawn raids in four cities, including london. they range in age from 17 to 28. what do we know about the target. we have a lot of details about the operation. what about the target?

    >> well, it doesn't appear they had a plot that far advanced. they have people under surveillance all the time. at this particular time they started to move on these 12 people.

    >> i said the suspects range in age from 17 to 28. young men suspected of plotting a terrorist attack .

    >> that profile remains constant of young males often of islamic orientation that have gone radical. that's still the profile of most terrorists. we're looking at four cities including london. do we know anything further about the plot?

    >> no. ea we don't know if they're tied together yet. the british cells are normally from the north area around manchester. this spreads out the geographic reach of the cells involved in britain and raises the concerns about british and european passport holders having access to terrorist activity .

    >> and the other thing that raises a concern, the time of the year. we know it was this time of the year where that young man suspected of hiding the bomb in his underwear, underwear bomber tried to detonate over the skies of detroit.

    >> that was on christmas eve last year. twhaps during the holidays, terrorism officials are always concerned because people gather in major groups for christmas festivities, tree lighting ceremonies. also on the first of the year. those are events that greatly increase the concerns of terrorism officials. this is probably why the british move now to try to preempt that.

    >> michael sheehan with the latest on this developing news.

    >> in a matter of minutes

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