Opposition politicians have vowed to fight the government's controversial plans to extend pre-charge detention to 42 days.
The counterterrorism bill received its second reading in the Commons unopposed last night but Liberal Democrats, Conservatives and some Labour MPs plan to attack specific passages when it enters the committee stage for scrutiny.
MPs remain deeply sceptical to the government's call to extend the period terror suspects can be held without trial to up to 42 days.
Labour backbencher and QC Bob Marshall-Andrews said there was "simply no evidence whatsoever" to back the bill and the shadow home secretary David Davis said there was "not one shred of evidence" in favour of extending pre-charge detention.
The government maintains the UK faces an "unprecedented" terror threat with increasingly sophisticated plots and the measure is the only means to improve security.
Opening the debate, home secretary Jacqui Smith said counterterrorism law must evolve as terrorism evolves or it will cease to be effective.
Ministers believe the measures in the counter-terror bill have been misrepresented and Ms Smith stressed the 42-day extension was a "wholly different model" to the government's failed attempt to introduce a 90-day limit.
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 crown prosecution service, and approved by the home secretary."
But Mr Davis accused ministers of conjuring up "nightmare scenarios" to justify the extension.
The shadow home secretary said the proposal gives up essential liberties without delivering any additional temporary security.
There is a line "a free country cannot cross without convincing justification," he told MPs.
Liberal Democrat home affairs spokesman Chris Huhne warned Ms Smith his party would fight the extension "tooth and nail".
He argued the bill threatened "hard won civil liberties".
Gordon Brown earlier defend the bill, arguing the idea the government was imposing a blanket 42-day detention is "a myth and that is wrong".
But human rights group Liberty remain opposed to the bill.
Director Shami Chakrabarti said: "All the spin about so-called safeguards has failed to soften up those who know the counter-productivity of detaining potentially innocent people for six weeks on suspicion alone."