The Counter Terror Bill has proved to be controversial
The government has been warned to expect tough opposition to its plans to extend the time limit on holding terror suspects without charge.
Its Counter Terrorism Bill was given an unopposed second reading but Lib Dems, Tories and some Labour MPs say they will fight parts of it later on.
Home Secretary Jacqui Smith insisted she wanted a "reserve power" not a permanent extension from 28 to 42 days.
But Chris Huhne said the Lib Dems would fight the proposal "tooth and nail".
The bill passed its first Parliamentary hurdle earlier when MPs gave it an unopposed second reading - now it goes to committee stage where MPs will scrutinise the detail.
Opening the debate Ms Smith said the proposal on pre-charge detention was a "wholly different model" to the planned 90-day limit that was defeated in the Commons in 2005.
She said the terrorist threat was of an "unprecedented scale" and the sheer weight and amount of material that had to be gone through - often of an international nature - presented a new challenge to investigators.
"As the threat from terrorism evolves, so our laws must adapt to remain effective," she said.
She said the government had moved a "considerable way" from the original 90-days option.
Ms Smith said: "We are not now proposing a permanent, automatic or immediate extension to pre-charge detention beyond the current maximum limit of 28 days.
"We are proposing a reserve power - not to be used lightly - that would mean that a higher limit could only become available if there was a clear and exceptional operational need, supported by the police and the CPS, and approved by the home secretary."
She said it would be subject to Parliamentary approval within 30 days, a senior judge would decide whether someone was held - and she pledged to continue a "consensus building approach" as the Bill continues its Parliamentary progress.
But some Labour backbenchers remain unconvinced by the arguments - and are expected to vote against the Counter-Terrorism Bill during the later stages of its passage through Parliament.
Labour's Bob Marshall-Andrews, a QC, said the government had produced "simply no evidence whatsoever" to back the Bill.
For the Conservatives, David Davis also told MPs: "There is not one shred of evidence for extension beyond 28 days - full stop."
While he said ministers conjured up "nightmare scenarios" - but there was already a solution, to use the Civil Contingencies Act.
And he said while the Conservatives would support parts of the Bill - such as allowing post-charge questioning - extending the pre-charge detention period up to 42 days could act as a "recruiting sergeant" for terrorists.
For the Liberal Democrats, Chris Huhne said aspects of the Bill were deeply intrusive to "hard won civil liberties".
He praised proposals such as the limited use of intercept evidence but attacked the proposal to allow pre-charge detention to be extended.
"We on these benches will fight tooth and nail against these provisions which we believe to prove to be a serious erosion of hard-won freedoms, just as crucially it will prove to be counter productive," he said.
But Labour former minister Frank Field criticised MPs who wanted to "remain passive and see what the terrorists inflict on us and then react accordingly".
Earlier Prime Minister Gordon Brown said: "There is no proposal for a blanket extension to 42 days. That is a myth and that is wrong."
The proposals are supported by some senior police officers - but could face a court challenge from the Equality and Human Rights Commission if passed.