LONDON, Aug 4: Police were on Wednesday questioning a dozen men held after a series of raids in London and across England triggered by intelligence on "alleged international terrorism".

A 13th man arrested in north London as part of Tuesday's raids has been released without charge, police said. The men, aged between 19 and 32, were suspected by police of involvement in the commission, preparation or instigation of acts of terrorism.

Witnesses described on Wednesday the dramatic daylight raids, as police continued to search a number of addresses in London, central England, and the northwestern town of Blackburn.

BBC television quoted police sources as saying the suspects were "all of (south) Asian origin" and that at least some of them were thought to be British nationals. It also said that Tuesday's raids were "not directly linked" to a crackdown on Al Qaeda suspects in Pakistan which prompted a heightened state of terrorist alert in the United States.

A Pakistani intelligence officer in Islamabad said on Wednesday that Britain had arrested a senior Al-Qaeda operative after a tip-off from Pakistani investigators. The official, speaking to AFP on condition of anonymity, could not say whether the man, identified as Abu Eisa Al Hindi, was among Tuesday's arrests.

The 13 suspects were taken to a central London police station to be questioned by officers of the Metropolitan Police's anti-terrorist branch. "Today's operation is part of continuing and extensive inquiries by police and the security service into alleged international terrorism," the police said in a statement on Tuesday.

On Sunday, the Home Office said Britain remained under a "real and serious" threat from terrorism, after the US authorities warned of potential truck or car bomb attacks against financial institutions in Washington and New York.

Tuesday's arrests came as Prime Minister Tony Blair came under pressure to spell out the level of the terrorism threat in Britain. The United States was put on a heightened state of alert following the seizure of computer files from Al-Qaeda suspects captured in Pakistan. -AFP