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

With three of the suspected attackers still at large, Britain's most senior police officer warned of more possible attacks and thousands of officers patrolled the country's rail network to reassure the public.

"There are many thousands of police officers trying to ensure the safety of Londoners," said Metropolitan Police Commissioner Sir Ian Blair (search) — three weeks to the day after the July 7 attacks that killed 56 people, including four homicide bombers.

He said it was a "a race against time" to find the suspected bombers.

"It does remain possible that those at large will strike again," he added. "It does also remain possible that there are other cells who are capable and intent on striking again."

British Transport Police (search) said it had scrambled the "largest ever deployment of police" to patrol the country's rail network.

"It is a time of heightened tension, and we have this deployment of police to give reassurance and deterrence," said spokesman Simon Lubin. He declined to say how many officers had been deployed.

Scotland Yard said the nine men were arrested under the Terrorism Act at two properties in the neighborhood of Tooting, south London, early Thursday. They were being held in a central London police station.

One of the four men suspected of carrying out the failed attacks July 21 was arrested in Birmingham, central England, on Wednesday. Yasin Hassan Omar, 24, was being questioned at a top-security police station in London.

Blair said the botched attacks, in which four bombs only partially detonated on three subway trains and a central London bus, was not a sign the terrorists had been weakened in any way.

"This is not the B team. These weren't the amateurs. They made a mistake. They only made one mistake, and we're very, very lucky," he said.

Blair said he was confident that police would find the bombers — as well as whoever backed them.

"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," Blair said.

Residents in Tooting said police had arrested three Turkish men who worked at and lived above a fast food restaurant selling halal burgers — made with meat slaughtered according to Islamic dietary laws.

The restaurant owner, who gave his name as Ali, declined to identify the men but said they were aged about 26, 30 and 40. The oldest man had worked for him for eight years, and the other two had started about two months ago, he said.

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."

Britain is home to many immigrants from the South Asian countries of Pakistan and India, among others.

Omar, a Somali citizen with British residency, was arrested in a dramatic raid by dozens of anti-terrorist police and bomb disposal experts on Wednesday in Birmingham, Britain's second-largest city. Omar is suspected of trying to blow up the Warren Street subway station on July 21.

ABC News, meanwhile, reported that British authorities investigating the July 7 attack had found 12 bombs and four improvised detonators in the trunk of the car of one of the suspected homicide bombers 35 miles outside of London five days after the deadly explosions.

The network broadcast photos — including one of a glass bottle apparently packed with explosives and covered in nails that could be used as shrapnel — and said they provided important clues about who was behind the attacks.

Other raids were carried out Wednesday in south London's Stockwell district, where officers arrested three women on suspicion of "harboring offenders." Detectives believe the man who tried to bomb a train near the Shepherd's Bush subway station on July 21 may have lived there.

Ali Reid, 37, who lives in the Stockwell apartment block that was raided, said he recognized the suspect from images released by police.

"I know the face. I used to see him coming and going," he said. Neighbors said he had three young children and lived with relatives.

A second July 21 suspect has been identified as Muktar Said Ibrahim, 27, also known as Muktar Mohammed Said. He came to Britain in 1990 from Eritrea, his family said. He was granted residency in 1992 and British citizenship in September 2004, the Home Office said.