British police arrested 13 men on Tuesday in a series of anti-terror raids across the country, one day after the United States issued a high alert warning. ?The men have been arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism,? a police statement said.

Britain had resisted calls to match the United States high state of alert after warnings that al Qaeda bombers might strike in America and played down concerns of an immediate security threat. On Monday, the United States tightened security and increased its alert level to ?High?, prompted by Pakistan?s capture of a suspected al Qaeda computer expert which yielded documents, computers, surveillance reports and sketches.

British newspapers said the material also indicated threats against unspecified targets in Britain. A police spokesman, however, declined to say if there was a link between intelligence from Pakistan and the raids.

The arrests of the men, in their 20s and 30s, were made in northwest London and in the southern English counties of Hertfordshire and Bedfordshire as well as the northwestern area of Lancashire.

British media reported that some of those detained were British citizens and all of Asian origin. About 500 people have been arrested in high-profile swoops in Britain under anti-terror laws brought in after the September 11, 2001, attacks in the United States but charges have been brought against about only 100 individuals. The operation was part of extensive inquiries by police and the security service into alleged international terrorism, said police. The police said searches were continuing and would take ?some time? to complete. Firearms officers were involved in some of the arrests but no firearms or hazardous substances had been recovered so far.

Blair?s government this week declined to follow the high-profile reaction in the United States in which police armed with assault rifles were deployed to protect financial centres in New York and Washington.

<B>Madeline CHAMBERS</B>