Meanwhile, authorities in Zambia have detained a British man sought in connection with the July 7 London bombings, a Zambian official said.
The official, who spoke on condition of anonymity, said Haroon Rashid Aswat, 31, had been detained in the Zambian border town of Livingstone after he crossed from Zimbabwe. The official declined to give any other details.
British investigators believe Aswat had been in phone contact with several of the four suicide bombers who killed themselves and 52 others in the attacks. The Los Angeles Times and other news media have reported that he was detained by Zambian authorities last week.
Metropolitan Police Commissioner Sir Ian Blair also warned that other terror cells could be planning attacks, and said the country faced "a somber moment."
"It does remain possible that those at large will strike again," Blair told a meeting of the Metropolitan Police Authority. "It does also remain possible that there are other cells who are capable and intent on striking again."
Scotland Yard police headquarters said nine men were arrested under the Terrorism Act at two properties in the neighborhood of Tooting, south London, early Thursday morning. They were being held in a central London police station.
One of the four men suspected of carrying out last Thursday's failed attacks 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 failed July 21 attacks, which came two weeks after 52 people were killed by four men who blew themselves up on three subway trains and a 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.
Meanwhile, police deployed the largest number of officers ever on Britain's rail network to reassure the public — three weeks on from the July 7 attacks that killed 56 people, including the four suicide bombers, and following last Thursday's botched attacks.
"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 in what he said was the "largest ever deployment of police."
Thursday's dawn raids brought to 20 the total number of people in police custody in connection with last week's failed bombings.