"I think it's perfectly reasonable for us in circumstances of great difficulty to have a greater detention in order that there can be the interrogation of people who are suspected of doing this," he told reporters.
"People rightly expect us to take the right measures to increase their security."
Blair earlier on Tuesday held a summit with opposition party leaders to discuss possible changes to anti-terror laws, including police demands to hold suspects without charge for up to three months.
Extension for questioning
Top police officials have called for an extension of the time terror suspects can be held to three months from 14 days.
There was an "obvious balance between the liberties of the subject and what the police need", Blair said.
"You would obviously have to have some sort of judicial oversight. We have to look at any proposal that they (police) make and they have made this proposal arising out of specific operation difficulties."
"So far as the three-month period of detention which the police have asked for, we see very considerable difficulties in that"
Conservative Party leader
British police are struggling to respond to a terrorist threat after two bomb attacks on the London transport system this month, one of which killed 52 people as well as four suicide bombers.
The prime minister has been keen to consult opposition parties to ensure bipartisan support for extending police powers.
Opposition Conservative Party leader Michael Howard said after the meeting all sides of politics must "work together" but warned that he had concerns about the increased detention.
"So far as the three-month period of detention which the police have asked for, we see very considerable difficulties in that," he said.