Coordinated anti-terror busts in and near London early Tuesday morning led to the arrest of eight men and the seizing of half a ton bomb-making material.

Sources said police believe the suspects may have planned to hit "soft" targets and that the operation may have been planned for several weeks.

The arrests were "a timely reminder that the [United Kingdom] and its interests abroad remain a target," Home Secretary David Blunkett said.

All the suspects were British citizens and were arrested as part of an operation targeting alleged international terrorist activity, said Deputy Assistant Commissioner Peter Clarke of the capital's Metropolitan Police (search) force.

Police confiscated over 1,000 pounds of ammonium nitrate (search) from a self-storage facility in west London, the largest seizure of bomb-making material since the Irish Republican Army (search) suspended its campaign in 1997.

Ammonium nitrate, which can be mixed with fuel oil to make a powerful explosive, was the key component in the bomb used to destroy the Oklahoma City federal building in 1995.

It was also used in the Oct. 12, 2002 blast in Bali, Indonesia, that killed 192 people, mostly Western tourists.

The British operation, which targeted residences and business properties, was not connected to the Madrid train bombs earlier this month or Irish republican terrorism, Clarke said. About 700 police officers took part.

The raids came after Metropolitan Police Commissioner John Stevens warned, in the wake of the Madrid bombings, that an attack on the United Kingdom by Al Qaeda was inevitable.

"We do know that we have actually stopped terrorist attacks happening in London but ... there is an inevitability that some sort of attack will get through, but my job is to make sure that does not happen," Stevens said, British news outlets reported.

The men, aged 17-32, were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.

Press Association, the British news agency, said all eight were of Pakistani descent, but police would not comment on that.

Clarke told reporters: "As we have said on many occasions in the past, we in the police service know that the overwhelming majority of the Muslim community are law-abiding and completely reject all forms of violence. We have a responsibility to all communities to investigate suspected terrorist activity."

Two suspects were arrested in Uxbridge, also in west London and three in Crawley, south of the capital. One was detained in Ilford, east London, another in Slough, west of London, and another in Horley, south of the capital.

Uxbridge and Slough are near Heathrow (search) airport, while Crawley and Horley are near Gatwick airport.

While police don't think Heathrow was a target, one of the suspects does work for a company that supplies food to the airlines, Sky News reported.

Officers conducted a total 24 searches that also targeted addresses in Reading, Luton and north London.

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

Ammonium nitrate was also used to make a bomb in a van that was parked near the U.S. Consulate in Karachi, Pakistan, on March 15, but did not explode; in a truck-bomb attack on  a British bank in Istanbul, Turkey in November; and, in years past, by the IRA in London and in Northern Ireland.

The Associated Press contributed to this report.