Prime Minister Tony Blair has urged Londoners to be calm and to get back to business as normal following a series of minor explosions on Thursday.
Mr Blair said there appeared to be no casualties in the incidents at three underground stations and on a bus.
He said the incidents were serious but stressed they were intended to scare, frighten and intimidate people.
Earlier Mr Blair held a meeting of the Cobra civil contingencies committee. He is now resuming his planned schedule.
Cobra, named after the underground Whitehall room in which it meets, coordinates the UK response to crises.
At his Downing Street news conference, 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.
Mr Blair again denied the London attacks were a result of the Iraq war and 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.
In the earlier incident Warren Street, Oval and Shepherd's Bush Tube stations were evacuated and lines closed after three blasts in what Met Police chief Sir Ian Blair said was a "serious incident".
In addition, a Number 26 bus in Hackney Road in Bethnal Green had its windows blown out by a blast. There were no injuries.