In one of the biggest anti-terror operations in the UK, half a ton of ammonium nitrate fertilizer was seized and eight Britons said to be of Pakistani descent were arrested early Tuesday.

According to agencies, over 800 policemen raided 24 premises in and around London early Tuesday morning. The fertilizer, often used in terrorist bombings, was recovered from a self-storage warehouse in west London.

According to the London Metropolitan police, the eight suspects picked up in London and its suburbs were suspected to be involved in the "commission, preparation or instigation" of acts of terrorism.

Deputy Assistant Commissioner Peter Clarke, head of the force's anti-terrorist branch, said the eight men were British nationals aged 17 to 32. The operation, which involved policemen from five districts, was not related to Irish terrorism nor the recent blasts in Madrid, he said.

Three of those arrested were from areas near Heathrow Airport, four near Gatwick to the south, and one from Ilford, east London, he said. Officers raided 24 houses during the raids, he said. "Part of the investigation will focus on the purchase, storage and intended use" of the ammonium nitrate.

"The threat from terrorism remains very real. The public must remain watchful and alert," he warned.

"While it is for the appropriate authorities to decide what action is taken against those individuals arrested, the fact that such action was felt necessary is a timely reminder that the United Kingdom and its interests abroad remain a target," Home Secretary David Blunkett said.

"We have always been clear with the people in the United Kingdom that we face a real and serious threat and have never disguised the fact that this threat could manifest itself in any number of ways," he added.