major police operation is under way on the UK's transport system, with officers on a precautionary high alert.
The effort to reassure the public and deter would-be attackers saw the rail system's "largest ever deployment", British Transport Police said. Metropolitan Police Commissioner Sir Ian Blair warned it was possible "that those at large will strike again", or that another cell could do so.Nine men have been held by anti-terror police in Tooting, south London.
On Wednesday night three women were held in Stockwell, a few miles away, on suspicion of harbouring offenders.
And as police questioned 21 July suspect Yasin Hassan Omar following his arrest in Birmingham, the search for the other three suspects continued, with a new picture of one of the men released.
'High visibility'
British Transport Police said the high alert was not in response to specific information.
It was called exactly one week after the failed 21 July attacks and three weeks after the 7 July bombings, in which 56 people died. Stations and trains were being patrolled by officers, although BTP would not say how many were involved.
Sir Ian Blair said he was confident the bombers would be caught, but the failure of the 21 July attacks did not mean a weakening of their capability or resolve."This is not the B team, these were not the amateurs, they only made one mistake and we're very, very lucky," he said. "The carnage that would have occurred, had those bombs gone off, would have at least been equivalent to those on 7 July."
'Ongoing investigations'
The arrests in Tooting involved raids on two addresses early on Thursday, Scotland Yard said.
Six men were held at the first address and three at the second. Searches were being carried out at both locations but police sources said no explosives had been found. All nine men were arrested under the Terrorism Act 2000 and taken to a central London police station for questioning. It is not thought that any of the suspected bombers is among them. The three women were arrested at about 1800 BST (1700 GMT) on Wednesday, after a raid at a flat at Blair House, close to Stockwell station in south London.
Tube security
The first of the suspected 21 July attackers to be held, Yasin Hassan Omar, was arrested in Heybarnes Road, in the Small Heath area of Birmingham, at 0430 BST on Wednesday.
The 24-year-old is suspected of the attempted bombing of a Victoria Line train, near Warren Street and was taken to London's high-security Paddington Green police station for questioning. Police hope the new picture of the suspected Shepherd's Bush attacker on a 220 bus, which is clearer than that previously available, will help to identify him. He boarded the bus in Shepherd's Bush at about 1320 BST and got off in Wandsworth at 1407 BST. The earlier image of the suspect showed him at Westbourne Park Tube station, wearing a short-sleeved dark blue England football shirt and dark trousers.
Police had previously named Muktar Said Ibrahim, 27 - also known as Muktar Mohammed Said - as a suspect from the attempted bomb on the Number 26 bus in Hackney.
They are trying to identify a man who carried out an attempted bombing at Oval.