Sky News reports at 10:00 A.M. local time that police have arrested 21 people in London, Birmingham and the Thames Valley. The arrest come after an extensive covert counter-terrorist operation that according to some reports may have began a year ago.

Deputy Commissioner Paul Stephenson gave a press conference outside of Scotland Yard and said that he was confident the police had disrupted a plot "to cause mass murder on an unimaginable scale".

U.S. officials have a more cautious outlook. "We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted," said U.S. Homeland Security Secretary Michael Chertoff.

Sky News reports that their "Crime Correspondent Martin Brunt said he had been told the threat was imminent and those arrested were mainly young, British-born Asian men."

Reuters reports that Independent terrorism expert Paul Beaver believes that the nature of the alleged plot suggested a connection to al Qaeda. "In the last two months al Qaeda promised that it would avenge Iraq and Afghanistan by attacking British and American aviation assets. I see a direct link with that," he said.

Terror threat levels have been raised both in the U.K. and U.S. The U.S. government has raising its threat assessment to "red," for commercial flights from the U.K. to the U.S. In the U.K. the security level had been raised from 'severe' to 'critical'.