The government says more time is needed to foil complex plots
The government has unveiled plans to let police hold terror suspects for up to 42 days without them being charged.
The Counter Terrorism Bill also proposes "full use of DNA" in investigations and greater use of post-charge questioning.
It advocates putting terror convicts on a register like that for sex offenders.
Some senior police officers support extending terror detentions beyond the current 28 days, but it is opposed by Lib Dems, Tories and rebel Labour MPs.
The bill says such powers should be used only "if exceptional circumstances require it".
It also demands stiffer sentences for other crimes where terrorism is a factor.
A survey by the Independent newspaper last month suggested 38 Labour MPs were against the 42-day detention plan - more than the 34 needed to defeat it.
TERROR DETENTION PLAN
Home secretary to get power to extend limit to 42 days if a chief constable and director of public prosecutions back it
Commons and Lords to have to approve extension within 30 days
If either votes against it, the power will come to an end by midnight that night
New power only available to police for two months unless renewed
Parliament to be recalled from summer recess if extension vote required
Detentions over 28 days to be approved by judge at least every seven days
Home Secretary Jacqui Smith, mindful of a potential rebellion, has been meeting backbenchers to press her case.
In a statement she said: "We all need to work together to tackle the terrorist challenge and we have consulted widely on the proposed measures.
"We have listened to the concerns of community groups and others and the proposals brought forward today aim to strike the right balance between the need to protect human rights and ensuring police have the powers they need, when they need them, to tackle terrorism."
Ms Smith added that the bill would work in combination with government efforts at "challenging extremist ideology".
Asked earlier on BBC Radio 4's Today programme whether she was proposing legislation to deal with a hypothetical situation, the home secretary replied: "We are putting in a provision for if it becomes unhypothetical."
Attempts to extend the limit to 90 days in 2005 ended in then prime minister Tony Blair's first Commons defeat.
His successor, Gordon Brown, has said he believes all parties accept there could be circumstances under which it is necessary to hold someone beyond 28 days - such as the complexities of investigating a multiple terrorist plot.
The government says it is better to act now in a considered way rather than find itself forced to respond to a future emergency
But the Conservatives and civil rights group Liberty argue that current powers to call a national emergency are sufficient when faced with a case that requires going beyond 28 days.
Shadow home secretary David Davis said: "The government has not been able to present a shred of evidence to justify extending pre-charge detention, and there is now a range of evidence pointing the other way.
How the rules would work: Home Office's thinking explained
"The government should focus on practical measures to get the best use out of the existing 28-day period - measures like post-charge questioning and the use of intercept evidence to prosecute those involved in terror."
Lib Dem leader Nick Clegg has said his party will join forces with the Tories to defeat the move in the Lords, if they do not manage to overcome Labour's majority in the Commons.
He said: "The obsession with 42 days is undermining, not supporting, the battle against terrorism.
"Ministers are taking their reliance on hypothetical examples to new lengths as they attempt to cover up for the complete lack of genuine evidence for such a move."
The Commons home affairs committee and the Joint Committee on Human Rights (JCHR) comprising MPs and peers have both come out against extending the terror limit.
Labour MP Keith Vaz, chairman of the home affairs committee, said: "We wouldn't support 42 days.
"What we do support is an emergency provision, which will allow us to build on the Civil Contingency Act."
An extension to 42 days would first have to be requested by a chief constable and a senior prosecutor - and would then be authorised by the home secretary.
Parliament would be formally notified, but in practical terms any individual could be held for 42 days subject to approval of a senior judge.
The Association of Chief Police Officers has pushed for an extension, saying more flexibility is needed.
But others, including England's top prosecutor Sir Ken MacDonald and former attorney general Lord Goldsmith, have raised doubts.
The register of convicted terrorists would hold personal details, such as home addresses, and it would be an offence not to keep police fully informed of any changes.
In the most serious cases the government envisages that people will be on the list for life.