CBS News Radio Correspondent Steve Holt reports over 700 officers took part in the raids in London and its nearby suburbs. All the suspects were British and were arrested as part of an operation targeting alleged international terrorist activity, Deputy Assistant Commissioner Peter Clarke said.
He added that the operation was not connected to the Madrid train bombs earlier this month or Irish republican terrorism.
Ammonium nitrate is a common fertilizer, but it can be mixed with fuel oil to make a powerful explosive. It was used in the Oct. 12, 2002 blast in Bali that killed 192 people, mostly Western tourists.
Clarke said the early morning raids targeted residences and business properties. The ammonium nitrate was recovered from a self-storage facility in west London, but there was no danger to people in the area, he said.
The men, aged 17 to 32, were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
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 airport, while Crawley and Horley are near Gatwick airport.
Officers conducted a total 24 searches that also targeted addresses in Reading, Luton and north London. There is an airport in Luton.
"The threat from terrorism remains very real," Clarke said. "The public must remain watchful and alert."
Clarke gave no details of the religious affiliation of the suspects, but he 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."
Ammonium nitrate was used to make a bomb in a van which was parked near the U.S. Consulate in Karachi, Pakistan, on March 15. It was defused before it could explode.
The substance was also used in a suicide bomb attack in which an explosives-laden truck was detonated outside a British bank in Istanbul, Turkey in November.
In years past, the Irish Republican Army used ammonium nitrate in attacks in London and in Northern Ireland. Ammonium nitrate was also used in the bombing of a government building in Oklahoma City which killed more than 160 people on April 19, 1995, and in the 1993 bombing of the World Trade Center.
Because of the decades long Irish insurgency, London authorities have far more experience dealing with terrorism than most of their American counterparts. In downtown London, for example, there are very few wastebaskets because of the threat of IRA bombs.
According to the Guardian newspaper, Tuesday's arrests were part of a crackdown launched after Sept. 11 in which hundreds of suspects have been detained. Muslim groups have complained that only 77 people have been charged out of 500 who were picked up.