New proposals to extend the period that terror suspects can be detained without charge are being planned by ministers.
Downing Street has said it is still considering options on extending police powers to detain terror suspects.
They would allow detention for up to 58 days - but with a range of safeguards designed to win over critics.
But it said the opposition had already "accepted the principle" that the government could use existing emergency powers to hold suspects for 58 days.
These safeguards include time-limiting the powers, and giving MPs and judges enhanced scrutiny over their use.
So the debate now was "a technical discussion" about safeguards, it added.
David Davis, for the Tories, said there was "not an ounce" of evidence for a plan which amounted to a "permanent undeclared state of emergency".
But the Tories say there is "not an ounce" of evidence for a law change which would amount to a "permanent undeclared state of emergency".
Stepped up
Terror suspects can currently be detained by police without charge for 28 days, but ministers and the government's overseer of terror laws, Lord Carlile, say the complexity of plots mean longer detention may be needed in future.
Ministers hope to move the debate on from the issue of the number of days BBC political editor Nick Robinson Read Nick's thoughts in full But opposition parties, some Labour MPs and civil liberty groups say they have yet to see evidence that the 28 day limit - brought in last year - is not long enough.
Mr Brown has said he wants consensus on terror law changes - and the idea of allowing post-charge questioning and the use of intercept evidence in terror cases has gained opposition support.
Mr Davis, for the Conservatives, said those changes would make any extension to the 28 day limit even less necessary.
He also said there were already powers to call a state of emergency, which would allow people to be detained without charge for 30 days on top of the 28 days.
'Permanent' emergency
But it appears that Mr Brown's plan is to bring in similar emergency powers but without having to declare a state of emergency.
Ministers are hoping to win over opponents by arguing that as the emergency power already exists to extend detention to 58 days, they are merely proposing safeguards to allow it to be done without needing a full emergency to be declared.
But Mr Davis rejected the proposals, saying the Conservatives could not support what amounted to a "permanent undeclared state of emergency".
If there were exceptional circumstances, such as a large number of simultaneous terror plots being probed by police, the Conservatives would back the use of temporary emergency powers under civil contingency laws to detain suspects.
'Evidence'
He also criticised security minister Lord West, who appeared to change his mind on extending detention after a meeting with Gordon Brown on Wednesday.
Mr Davis said the peer's "grovelling retraction" showed the issue was about political considerations rather than genuine security needs.
Lord West's apparent change of stance - he said he had merely got his words wrong initially - was called "Keystone Cop" politics by the Lib Dems.
And David Winnick, a Labour MP opposed to any extension, told Today that he had not been persuaded to back the proposals, despite the outlined safeguards.
Ministers and the government appointed overseer of terror laws, Lord Carlile, have all said they believe the 28 day limit will need to be extended because of the growing complexity of cases being investigated.
The issue of how long terror suspects can be detained without charge led to Tony Blair's first Commons defeat as prime minister, when he sought to increase the limit to 90 days.
