Under the Terrorism Bill, suspects can be detained without charge for up to three months.
Mr Blair said arguments made by the police for detaining suspects for 90 days instead of the current 14 days were “absolutely compelling”.
The legislation also calls for a ban on those who glorify terrorism, sell extremist books, receive or provide terrorist training, and prepare to commit attacks.
The laws were drafted in the wake of the July 7 bombings on London.
Critics slam legislation
But opposition parties, human rights activists and Muslim fear the changes will trample on civil liberties and fuel community anger.
Amnesty International denounced the bill as “clumsy, dangerous and ill-conceived” and said the proposed detention period as draconian.
It claimed that it handed a victory to the July bombers by crushing freedoms built over centuries.
Debate on the bill is due to begin formally on Thursday in the House of Commons.
On the eve of its introduction to parliament, Mr Blair was sticking to his guns on some of the most controversial proposals.
“What I have to do is to try to do my best to protect people in this country and to make sure that their safety and their civil liberty to life come first, and that is what I’m going to try to do,” Mr Blair told parliament.
Addressing Mr Blair in the House of Commons, Charles Kennedy, the leader of the opposition Liberal Democrats, said the detention plan was “wrong” and questioned whether there was even a consensus within the Labour government.
Home Secretary Charles Clarke, who was in Europe on Wednesday, appeared open to concessions on the detention plan during a grilling by a parliamentary committee on Tuesday.
London’s mayor Ken Livingstone expressed some reservations.
Mr Livingstone said any laws must pass what he called “Nelson Mandela test.”
The mayor said the changes must not be so tough that Mr Mandela, the activist who became South Africa’s first black president, and his supporters would have been banned under them.
The government’s own independent terror watchdog, Lord Alexander
Charles Carlile of Berriew, also expressed concerns.
Lord Alexander said journalists could be caught in the dragnet by simply attending a terrorist training camp.
He added that the extension of the detention period could be vulnerable to a challenge under the Human Rights Act.
However, he conceded that there had been occasions when plots had escaped prosecution because of existing time limits.