A major police operation is under way on the UK's transport system, with officers on a precautionary high alert.
Police think the Shepherd's Bush suspect took off a blue England shirt
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 Yassin 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.
British Transport Police said the high alert was not in response to specific information.
Raids took place close to Tooting Broadway Tube station
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."
The arrests in Tooting involved raids on two addresses early on Thursday, Scotland Yard said.
WHO IS BEING HELD?
28 July - Nine men arrested in Tooting
27 July - Three women arrested in Stockwell
27 July - Man arrested in Small Heath, Birmingham
27 July - Three men arrested in Ward End, Birmingham
25 July - Man arrested near Curtis House, north London
24 July - Another man arrested near Curtis House
23 July - Man arrested in Tulse Hill, south London.
22 July - Two men arrested in Stockwell, south London. One was released on 27 July and the second is still being held
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.
The first of the suspected 21 July attackers to be held, Yassin 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.
The Shepherd's Bush suspect was previously shown at Westbourne Park
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 Ibrahim Muktar Said, 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.
TRACKS OF THE BOMB SUSPECTS
All journeys started between 12:20 and 12:25. Times approx.
OVAL: Man boards northbound Northern Line train at Stockwell and tries to set off bomb between Stockwell and Oval, where he leaves the train. He is chased out of the station at 1235 BST, but escapes towards Brixton.
HACKNEY: Man, identified by police as Ibrahim Muktar Said- or Muktar Mohammed Said - also sets off from Stockwell. Boards Number 26 bus at 1253 at Bank. Police believe he was carrying bomb in a grey and black rucksack, and tried to detonate bomb while on board. Gets off in Hackney Road, near junction with Columbia Road, at 1306.
WARREN STREET: Man who police believe is Yassin Hassan Omar boards Tube train at Stockwell carrying a purple rucksack that they say contained a bomb. Later tries to set off bomb on a northbound Victoria Line train between Oxford Circus and Warren Street, detectives say. Seen without rucksack at 1240 in Warren Street Station before running towards exit and vaulting over ticket barriers.
SHEPHERD'S BUSH: Man wearing dark blue baseball cap and carrying small rucksack enters Westbourne Park Tube station and gets a train travelling towards Shepherd's Bush on Hammersmith & City Line. Tries to set off bomb at 1225 before escaping, probably through window at the end of the carriage, and running along the tracks.