London police have arrested nine more people in connection with last week's botched attempt to bomb the city's transport system. At the same time, police have conducted a massive security operation three weeks after four British Muslim suicide bombers killed 52 passengers on subway trains and a bus.

Police made the arrests at two houses in the south London neighborhood of Tooting. Authorities say the arrests were linked to the failed attempts last Thursday by four men to blow up three subway trains and a bus.

Only one of the four prime suspects is in custody, and police say none of those arrested in the Tooting raids are among the fugitive would-be bombers.

Police also have mounted their biggest-ever security operation on London's subway system, given that the fatal attacks of July 7, and the bungled attempts of July 21, both occurred on a Thursday.

Uniformed transit police, backed by armed officers and plainclothes undercover agents were deployed across the 275-station subway network, which carries some three million passengers a day.

London Police Commissioner Ian Blair says security is high because authorities fear more terrorist cells capable of major attacks are operating undetected.

Mr. Blair says London barely escaped a major tragedy last week.

"The second attacks, on the 21st of July, should not be taken as some indication of the weakening of the capability or the resolve of those responsible," he said.  "This is not the B-Team. These were not amateurs. They made a mistake. They only made one mistake. And we're very, very lucky."

Police continue to question 24-year-old Yasin Hassan Omar, who police say attempted to blow up a subway train near the Warren Street station last Thursday. He was captured by officers who brought him down with a stun gun in a raid on a house in the central English city of Birmingham early Wednesday.

Police are also seeking a 27-year-old man, Muktar Said Ibrahim, also known as Muktar Said Mohammed, who authorities say tried to blow up a bus. Police have released the photos, but not the names, of two other suspects and they are making a nationwide appeal for information about their whereabouts.