LONDON — Counter-terrorism police arrested eight men suspected of preparing a major bombing in Britain and seized half a ton of ammonium nitrate, which authorities said could have been used to make a massive fertilizer bomb.
The arrests Tuesday sent a ripple of fear through Britain, coming after this month's train bombings in Madrid -- blamed on a Moroccan group linked to Al Qaeda -- and on the same day that authorities in the Philippines said they had thwarted a major attack there.
In the largest counter-terrorism operation in Britain since Sept. 11, 2001, about 700 police officers raided two dozen homes and businesses in London and elsewhere in southeastern England. The raids fueled speculation that an attack might have involved travelers at the country's two busiest airports. One of the men detained was arrested at a Holiday Inn near Gatwick Airport, and others lived near Heathrow Airport.
Some of the raids also took place in Luton, site of an airport used by budget travelers to Europe, and, according to German intelligence documents leaked last year, the site of a suspected terrorist cell linked to Abu Qatada, a detained radical Muslim cleric.
The suspects are Britons of Pakistani descent, according to media reports, although police would say only that they were British citizens.
Britain's top police officer recently warned that a terrorist attack in the country was inevitable, even though undercover police have been deployed to train stations, subway lines, airports and other places where large crowds gather. Authorities had launched a campaign to make Britons more aware of the reported threat. The large amount of potential explosives suggested that the plot was more advanced than any previously discovered scheme by Islamic groups in Britain, authorities said.
"This is the first major non-Irish terror case where you have guys who were clearly going to do something, to plant bombs," a British law enforcement official said. Investigators believe that the suspects intended to carry out an attack similar to the bombings in Madrid, which killed nearly 200 people on commuter trains.
"It was going to be a plot of great size," said a European police official, who asked to remain anonymous. "They were going to go after a civilian target, soft targets, places where there are a lot of people."
BBC security correspondent David Gardner quoted unnamed sources as saying that the arrests came about through a successful infiltration of a terrorist group by undercover agents from MI5, Britain's domestic intelligence service, and the sources believe that police arrested the group's ringleader.
"There is an air of restrained jubilation," Gardner said of the arrests.
The Madrid attacks were Europe's worst terrorist strike in 15 years. In the aftermath, police throughout Europe stepped up monitoring of suspected Islamic extremists.
Spanish investigators say the trains in Madrid were chosen to maximize the number of victims -- a departure from Al Qaeda's trademark attacks on symbolic targets such as embassies or landmarks. Officials fear that such attacks might be repeated elsewhere in Europe.
Deputy Assistant Commissioner Peter Clarke of the Metropolitan Police, Britain's national coordinator of anti-terrorism investigations, announced the arrests in a rare appearance before reporters. He said the eight arrested were in custody "on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism."
Some Islamic groups expressed anger at the raids, saying Muslims had been unfairly targeted. They said hundreds of Muslims had been detained and questioned in Britain in recent years, most of them freed without charge. In all, 529 people have been arrested under the country's anti-terrorism laws, including 77 charged and seven convicted.
"This is the kind of heavy-handed tactic that we have come to expect from the police," said Anjem Choudary, a British attorney and Islamic activist who teaches at the London School of Sharia. Saying the ammonium nitrate could have been used for nonviolent purposes, Choudary urged skepticism toward police statements about those arrested. "There is no evidence that any of them really are involved in terrorism," he said.
Clarke took pains to say that the crackdown was not directed against Muslims.
"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," he said.
But he added: "We have a responsibility to all communities to investigate suspected terrorist activity.... The threat of terrorism remains very real. The public must remain watchful and alert."
The ammonium nitrate was found under a plastic covering at a nondescript 24-hour storage facility in west London. It formed a bundle about 6 feet high and 2 feet thick.