LONDON, March 30: Police arrested eight men and seized a cache of explosives during raids at dawn on Tuesday in Britain's biggest anti-terror operation since the Sept 11, 2001, attacks.

With Europe already on a high state of alert after the Madrid bombings, British police pounced in 24 separate raids, seizing more than half a ton of ammonium nitrate fertilizer, which is prime bomb-making material.

The eight suspects were believed to be of Pakistan origin, police sources said. There was enough explosive material to launch an attack as big as the devastating Irish Republican Army bomb that hit London's financial district in 1996 and killed two people.

The fertilizer was similar to that used in the 2002 Bali bombings - but there was no clue about possible targets. Britain has long feared it could be a prime target for Muslim militants.

London's police chief has repeatedly said he believes an attack is inevitable. Britain has been on high alert since the Sept 11 attacks in New York and Washington.

Peter Clarke, head of Britain's anti-terror branch, said at a news conference that the fertilizer was discovered in a two-metre high plastic bag in a west London warehouse.

"Part of the investigation will focus on the purchase, storage and intended use of that material," Mr Clarke said. But he did stress the operation was not linked to investigations into the coordinated train bombings in Madrid on March 11, which killed nearly 200 people, or to Irish extremists. -Reuters