In Pakistan, an intelligence official said the arrest of an Islamic militant near that border several weeks ago played a role in "unearthing the plot." The official, who spoke on condition of anonymity, said some suspects arrested in Britain were linked to al-Qaida. However, authorities stopped short of accusing al-Qaida directly for the plot.

A senior Pakistani government official, also speaking on condition of anonymity because he was not authorized to comment on the matter, said "two or three local people" suspected in the plot were arrested a few days ago in the Pakistani cities of Lahore and Karachi.

French Interior Minister Nicholas Sarkozy said the group "appears to be of Pakistani origin," but did not give a precise source for the information. Britain's Home Office refused comment.

A British police official, speaking on condition of anonymity because of the sensitivity of the investigation, said the suspects were "homegrown," though it was not immediately clear if all were British citizens. He said authorities were working with Britain's large South Asian community.

Tariq Azim Khan, the Pakistani minister of state for information, said "these people were born and brought up in the United Kingdom. Some of them may have parents who were immigrants from Pakistan."

Raids were carried out at homes in London, the nearby town of High Wycombe and in Birmingham, in central England. Searches continued throughout the day, and police cordoned off streets in several locations. Police also combed a wooded area in High Wycombe.

Hamza Ghafoor, 20, who lives across the street from one of the homes raided in Walthamstow, northeast of London, said police circled the block in vans Wednesday and that they generally swoop into the neighborhood to question "anyone with a beard."

"Ibrahim didn't do nothing wrong," Ghafoor said, referring to a suspect. "He played football. He goes to the mosque. He's a nice guy."

The British government raised its threat assessment to its highest level — critical — which warns that a terrorist attack could be imminent. The U.S. government, following suit, raised its threat assessment to red alert, also its highest level, for commercial flights from Britain to the United States.

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Associated Press Writers Pat Milton and Tom Hays in New York, Lara Jakes Jordan in Washington and Jay Lindsay in Boston contributed to this report.