Police say there were raids on 24 addresses in and around London early Tuesday in one of Britain's biggest anti-terrorism operations since the 2001 attacks against the United States.

The chief anti-terrorism officer of the Metropolitan Police, Peter Clarke, said a raid in west London had found half a ton of ammonium nitrate, which can be made into a bomb. He said police have arrested eight men, all British citizens ranging in age from 17 to 32.

"Today's operation, which involved some 700 officers in total, is part of continuing and extensive inquiries by police and the security service, into alleged international terrorist activity," said the police official. "And I must stress that the threat from terrorism remains very real."

Mr. Clarke did not comment on news reports that identified the arrested men as suspected Islamic terrorists, but he did say British Muslim leaders had been contacted.

"We in the police service know that the overwhelming majority of the Muslim community are law abiding and completely reject all forms of violence," he stressed. "We have a responsibility to all communities to investigate suspected terrorist activity. Today, at both the local and national level, we have been holding discussions with community leaders and other representatives, to address any concerns they may have."