Prime Minister Gordon Brown is pressing ahead with controversial plans to extend the time terror suspects can be held without charge to 42 days.
The government says more time is needed to foil complex plots
Home Secretary Jacqui Smith is due to meet backbench Labour MPs on Monday to try to win over opponents.
But Lib Dem leader Nick Clegg has said his party will join forces with the Tories in the Lords to defeat the move.
Mr Clegg predicted Mr Brown would suffer the first defeat of his premiership on the issue.
But the prime minister said he believed it was still possible to reach a cross-party consensus when the government publishes its Counter Terrorism Bill later this week.
He claimed all the main parties accepted there could be circumstances in which suspects would need to be detained beyond the current 28-day limit.
"It may be a multiple terrorist plot, it may be a whole series of complex investigations that need to be done in the context of there being major terrorist incidents," Mr Brown told Sky News.
"I believe that it is possible to build a consensus on that."
He acknowledged that the government needed to offer assurances that civil liberties would be protected.
"Can you give the right assurances that the civil liberties of any individual will not be trampled upon? Can you give an assurance that there will be no arbitrariness in the way people are treated?" he said.
But Mr Clegg said the government's offer of greater parliamentary oversight was a "false concession" .
"When you look at the detail...it leads to the absurd position that you could have someone locked up for that longer period of time and Parliament would only be able to decide on whether that was justified or not after that longer period of detention has already expired.
That is not a meaningful concession," he told BBC One's the Andrew Marr show.
He said the government was "sailing pretty close to the wind" when it came to getting the plans through the Commons, with many of its own MPs expected to rebel.
But he said the move would in any case be defeated when it reached the Lords, where the Lib Dems and Tories outnumber Labour peers.
Jacqui Smith said the government had already moved "quite considerably" in response to the criticism of the proposed legislation.
She said it now recognised that any legislations should not be a "permanent" change and should be "time-limited".
But she said the government wanted a "reserve power if the circumstances necessitated, it would allow an application to hold somebody for longer than 28 days."
"The 42 days is not some sort of target - it's a safeguard, it's an absolute maximum."
She said that it was better for the government to act now in a considered way rather than find itself forced to respond to a future emergency.
Previous attempts to extend the limit have proved controversial and moves to extend it to 90 days in 2005 were defeated by a combination of Tory, Lib Dem and some Labour MPs.
The new proposals would involve the House of Commons and the House of Lords retrospectively being asked to approve any decision to hold an individual for longer than the 28 days, up to the 42-day maximum.
High profile figures, including the former lord chancellor, Lord Falconer, ex-attorney general Lord Goldsmith and the director of public prosecutions, have criticised the plans.
And two Parliamentary committees - the Home Affairs and the joint committee on human rights - have both come out against extending the terror limit.
In response to Mr Brown's comment, human rights charity Amnesty International said it would "fly in the face of basic human rights".
Its UK director Kate Allen said: "The case for extending pre-charge detention has been widely rejected.
"We need an approach to counter-terrorism that respects people's rights - not one that will cause more division and alienation."