"It does remain possible that those at large will strike again. It does also remain possible that there are other cells that are capable and intent on striking again," Blair told a police authority meeting.
Scotland Yard said the nine men were arrested under the Terrorism Act at two properties in the neighbourhood of Tooting, south London, early on Thursday. They were being held in a central London police station.
The arrests bring to 20 the number of people police have in custody, including one of the alleged bombers.
Also on Thursday, a London Underground railway station was evacuated and part of a main east-west line closed in a security alert.
A British Transport Police spokeswoman said Southwark station was closed and Jubilee Line services suspended between Waterloo in central London and Canary Wharf in the east London business district.
She said she had no more details. London police said they had no immediate information.
Police have dealt with dozens of security alerts across London since the 21 July attempted bombings which came exactly two weeks after a team of bombers killed 52 people in the capital.
Police say four British Muslims carried out the 7 July bombings, which they have linked to al-Qaida.
On the second group of attackers still at large, he said: "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."
His statement comes as British police have interrogated their first captured London bombing suspect and arrested nine more men as details emerged of a large-scale terrorist battle plan.
Police disabled Somali-born Yasin Hassan Omar with an electrical charge from a Taser stun gun to overpower him in a raid on Wednesday in the central English city of Birmingham.
Suspected of an attempted bombing on a London Underground train near central Warren Street station on 21 July, his interrogation at London's high-security Paddington Green station may be pivotal to unmasking the scale of the terror plot.
Three other suspected bombers were still on the run after the botched attempt on 21 July to blow up three London Underground trains and a double-decker bus, a mirror of the 7 July blasts that killed 56 people including the four attackers.
As the hunt gained ground on Thursday, London police
As many as 16 bombs were
found at a station in Luton
arrested nine men at two properties in south London over the 21 July attacks. Police sources said none was believed to be an actual bomber.
Police flooded the Underground system.
"This is certainly the largest number of people we have had at stations, at Underground stations," London transport police spokesman, Simon Lubin, said. "I would say (the largest) ever."
Exactly one week after the attempted 21 July blasts, Lubin said the aim was both to reassure the public and deter the on-the-run bombers.
"The state of alert hasn't changed. The operational deployment is to get as many people out there as possible," he said. Lubin said leave had been cancelled for some officers since the first attack, and re-inforcements drafted in from around Britain.
Many police, some armed, could be seen at London Underground stations - six alone outside the central Gloucester Road station.
Only one of the 21 July suspects
has been arrested
Even with Hassan Omar in captivity, police are racing to capture the rest of the gang to prevent a new attack.
"I must emphasise that until these men are arrested they remain a threat," said Peter Clarke, the Metropolitan Police's anti-terror chief.
Only one of the other fugitives has been named - 27-year-old Muktar Said Ibrahim, also known as Muktar Mohammed Said, who migrated to England from Eritrea as a child.
Police released pictures caught on closed circuit television of the other two suspects, including a fresh image revealed on Wednesday of the man accused of trying to detonate a bomb on a train in west London on 21 July.
Police released a new picture
of a bombing suspect
The suspect, who had a shaved head and short beard, was pictured on board a bus travelling to south London after the bungled attacks.
He was wearing dark trousers, a white vest and a wristwatch on his left arm.
Adding to the sense of urgency, British newspapers widely picked up on a report by the US television station ABC that the 7 July bombers left a car packed with up to 16 bombs, raising fears of a large scale campaign.
Police at Scotland Yard refused to comment on the X-ray photographs of a bottle-shaped, nail-studded bomb that were plastered across the front pages of most dailies after being leaked to the ABC.
"How big was the terror plot?" asked the left-leaning Independent newspaper next to a large picture of the device.
The British press speculated on
the size of the bombing plot
"The terrorist cell ... may have been planning to throw nail bombs into a nightclub or a football crowd," it said in a page one news story.
The 16 bombs were found in a rental car abandoned at a railway station in Luton, north of the British capital, by the four bombers, who boarded trains to King's Cross exactly three weeks ago, newspapers reported.
Some of the homemade explosives, thought to be a mix of acetone-based chemicals, were in bottle shaped containers with dozens of nails packed around them and held in place by cling film.
The nails are designed to act as shrapnel. Others were flat, pancake-shaped bombs, the Guardian said.
All the bombs appeared to have
been made by the same person
The Times newspaper said the bombs used by the first squad and the four would-be bombers two weeks later appeared to have been manufactured by the same person.
"The nature and number of bombs points to the existence of a large and well-equipped terrorist cell intent on a sustained campaign of attacks," it said. Police have yet to directly link the 21 July and 7 July attacks.
In a complex web of investigation, police arrested three more men in a raid at a second home in east Birmingham on Wednesday, while three women were also detained in London on suspicion of "harbouring offenders", police said.
A total of 17 people remained in custody in relation to the 21 July plot.
Police chiefs have urged Prime Minister Tony Blair to extend the detention time to up to three months, but The Independent newspaper said the prime minister favoured allowing rolling two-week periods although no decision had been made.