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

The men - whose ages range from 17 to 28 - 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 the alleged plot appeared to be significant and involved several British cities, but he did not name them.

Police have 28 days to charge the men or free them.

Possible targets scouted include the Houses of Parliament in London and shopping areas around Britain, according to a security official who spoke on condition of anonymity because the investigation has not concluded.

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

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

European officials typically step up security around the holidays. They have done so especially this year 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.

Germany upped its terror alert Nov. 17 in response to a perceived new threat from Islamist extremists. Berlin's interior minister, Ehrhart Koerting, said recently the threat had eased.

Italian officials on Tuesday were investigating whether an explosive device found in a subway car in Rome was terror-related. Bomb-disposal experts concluded the device could not have exploded.

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

Monday's raid, a joint operation by Britain's domestic spy agency MI5 and police, was the largest since one in April 2009, when 12 men were detained over an alleged al-Qaeda bomb plot in Manchester.

All were released without charge and ordered to leave Britain. One, Abid Naseer, was later rearrested at the request of the United States, which says he was involved a plot to bomb the New York City subway. He is in jail awaiting extradition.