Downing Street says it has no intention of giving a "running commentary" on the on-going police operation in London.
Mr Blair is being briefed on developments
Prime minister Tony Blair is at Chequers and is being kept informed about developments on a regular basis.
He had a meeting earlier with key officials from the emergency COBRA committee, a spokesman said.
The comments come after a man was shot dead by armed officers at Stockwell Tube station, in South London, and police hunt four would-be bombers.
A spokesman said the investigation into Thursday's explosions was a matter for the police and the emergency services.
Parliament has risen for an 80-day summer break. Calls in the press for MPs to be recalled have so far not been heeded.
On Thursday, Mr Blair urged Londoners to be calm and to get back to business as normal following a series of minor explosions.
He said the incidents were serious but stressed they were intended to scare, frighten and intimidate people.
Cobra, named after the underground Whitehall room in which it meets, coordinates the UK response to crises.
At a Downing Street news conference on Thursday, alongside Australian prime minister John Howard, Mr Blair said: "I have just spoken to the Metropolitan Police Commissioner and his hope is that things can get back to normal as soon as quickly as possible.
"We can't minimise incidents such as these, all I would like to say is this - we know why these things are done, to frighten people and make them anxious and worried.
"Fortunately in this instance there appears to have been no casualties. We have just got to react calmly."
Mr Blair denied the attacks were a result of the Iraq war.
He said the "roots of this are deep" and terror attacks went back more than a decade.
He said terrorism would only be defeated by "going after the ideas of these people ... taking them on and defeating them".
"It doesn't change us. It is not going to change what we do. To react in any other way is to engage in the game they want us to engage in."
Mr Blair said he hoped it would quickly become apparent who was behind the latest attacks.
The prime minister went on to hail the spirit of Londoners whom he said were "canny enough" to know what "these people are trying to do". London was "united" he added.
Liberal Democrat leader Charles Kennedy meanwhile issued a statement praising the "calm and professional" response of police and other emergency services.
"What comes through loud and clear is the extent to which all the public agencies and all of us as members of the public are in this together for as long as it takes to see off these threats to our normal way of life," he said.
Shortly before the press conference, armed police outside Downing Street were seen arresting a man at gunpoint, ordering him to remove a rucksack and open his shirt before he was led off. He was one of two people arrested in Whitehall.