Tony Blair has warned it would be "irresponsible" to ignore police calls to let them hold terrorist suspects for up to 90 days without charge.
Mr Blair said he had a duty to make the UK safe
At his monthly news conference, he denied criticism from civil liberties groups that he was "authoritarian".
But the Tories criticised Mr Blair's "aggressive" approach and the Lib Dems also still oppose the detention plans.
Earlier, Home Secretary Charles Clarke said the 90 days were "not God-given" and admitted consensus was unlikely.
He also said terrorism could now not be justified anywhere in the world.
The government publishes its Terrorism Bill on Wednesday.
BBC political editor Nick Robinson said there were likely to be concessions from ministers - particularly to win over Labour backbenchers.
The detention plans are proving the most controversial issue.
Currently, suspects can be held for up to 14 days without charge.
Mr Blair said the police had provided good evidence for extending the time limit - such as the complex nature of terror cases, which involved gathering large amounts of evidence.
He continued: "If they are right, then how can I responsibly refuse to something that will actually protect - as I say - the most basic civil liberty which is the right to life?"
He was questioned after the new top judge in England and Wales, Lord Phillips, warned politicians not to interfere with the judiciary or browbeat judges.
Mr Blair said he was not "browbeating" judges but he was in the "decision making seat".
"All I'm saying to the judiciary is be aware there's a proper role for the judiciary and a proper role for Parliament," he said.
The prime minister argued that terrorist activity was "of a wholly different order" from any before.
"We need to make sure therefore that we give ourselves every possible opportunity to prevent such terrorist acts occurring," he said.
Some senior lawyers have warned the detention plans could fall foul of human rights laws.
After hearing both Mr Blair's and Mr Clarke's comments, Conservative shadow home secretary David Davis suggested the government was not speaking be with "one voice".
"The prime minister's aggressive stance seems rather at odds with the constructive approach of the opposition parties to achieve both the security and the liberty of the British public," he said.
Lib Dem leader Charles Kennedy said Mr Blair had still failed to make the case for extending the detention time limit.
"It would be far better to charge suspected terrorists with a lesser offence and hold them on remand while investigations are ongoing," said Mr Kennedy.
In earlier heated exchanges, Mr Clarke told the Commons home affairs committee said he had tried to be "flexible" over the terror laws.
But he acknowledged there was concern over the detention plan.
The home secretary said it was important to hold people "absolutely determined to engage in terrorist acts".
He said he could not "think of a situation in the world" where terror was justified for political change.
Committee chairman John Denham said this "presumably" meant Iraq was "the war to end all wars".
Mr Clarke replied: "It's not terrorist violence. This is about terrorism, not about violence used in the way that you describe.
"Those who use terrorism to make progress in the Middle East are wrong."
The Terrorism Bill will also outlaw indirect support or glorification of terrorism.
Mr Clarke said there had been a "fantastic transformation" in the world towards democracy in recent decades, including the former Soviet Union, South Africa and Latin America.
He said: "My argument is that we are moving to, and have moved to, a different political era."
Labour MP Dave Winnick asked whether anyone who supported Nelson Mandela's African National Congress would have been prosecuted had the proposals been in force during the apartheid era.
Mr Clarke would only reply that people would not have been guilty merely by not condemning the ANC.