LONDON - Anti-terrorist officers arrested nine men in raids early today in connection with the botched July 21 attacks on London's transit system, bringing to 20 the number of people police have in custody, including one of the alleged bombers.

Scotland Yard police headquarters said the nine were arrested under the Terrorism Act at two properties in the neighborhood of Tooting, in south London.

The arrests follow a significant breakthrough on Wednesday, when authorities in the central England city of Birmingham arrested one of the four men suspected of carrying out the failed attacks - Yasin Hassan Omar, 24. He was being held at a top-security police station in London.

Metropolitan Police Commissioner Sir Ian Blair, meanwhile, warned today that the terrorists behind the bombings - or even other terrorist cells - could strike again.

"The second attacks on July 21 should not be taken as some indication as a weakening of the capability or the resolve of those responsible," said Blair. "These weren't the amateurs. ... They only made one mistake and we are very, very lucky," he added, referring to the fact the July 21 bombs only partially detonated.

"The carnage that would have occurred had those bombs gone off would have at least been equivalent of those on July 7 and therefore it is absolutely imperative that we find those responsible," he added.

Peter Clarke, the head of London's police anti-terrorist unit, called Omar's arrest "an important development in the investigation." But he warned that the three remaining bombers still on the run presented a danger.

Security was tight at many subway stations in central London today - the three-week anniversary of the July 7 attacks on three subway trains and a bus that killed 56 people, including the four suicide bombers. today was also the one-week anniversary of the failed July 21 attacks.

Police also deployed the largest number of officers ever on Britain's rail network to reassure the public. "It is a time of heightened tension, and we have this deployment of police to give reassurance and deterrence," said Simon Lubin, a spokesman for the British Transport Police.

He declined to say how many officers were taking part.

Residents in Tooting said police arrested three men who lived in an apartment above a takeout restaurant selling halal burgers. Halal is meat from a herbivore slaughtered in a humane way - as Islam requires.

The restaurant owner, who gave his name only as Ali, said the men who lived there did not seem to be suspicious in any way.

Six other men were arrested from a property in nearby Garratt Terrace, a street opposite the Tooting Broadway subway station.

"There were about a dozen armed police officers shouting, 'Come on out or we'll send the dogs in.' And then I saw one large, older-looking Asian man being led out. He was dressed in a white gown or robe," said local resident Ben Astbury, 25, who watched the raid from his house in Garratt Lane.

"After that I saw about three other men but I couldn't see clearly what they looked like. My girlfriend then saw two more."

Omar, a Somali citizen with British residency, was arrested in a dramatic raid by dozens of anti-terrorist police and bomb disposal experts, some in heavy body armor.

Interrogations of Omar may be key to determining whether last week's failed attacks are linked to the July 7 bombings. Omar is suspected of trying to blow up the Warren Street subway station last Thursday.

Kati Stewart, 31, a health care worker who lives across the street from Omar in the Small Heath neighborhood of Birmingham, said she'd seen four men coming and going frequently over the past two weeks. "They would come at 2 a.m., and then when you looked in the morning, the car had gone," she said.

But Omar, a refugee from Somalia who came to Britain in 1992, generally attracted little attention in the diverse neighborhood, where residents of many ethnic backgrounds and faiths - Indian, Pakistani and Irish; Christian, Hindu and Muslim - say they live together peacefully.