Ministers do not need to change the law on how long terror suspects can be held before being charged, say the Tories.
Acpo says the current detention limit is not long enough
Shadow home secretary David Davis said the 28-day limit should remain and talk of a 90-day limit was "a distraction".
He said ministers had emergency powers to extend it, in a crisis, by 30 days under the Civil Contingencies Act.
Police say they are "up against the buffers" on the current limit, but the Lib Dems and Tories say there is no evidence it needs to be extended.
Attempts to extend the pre-charge detention period from 14 to 90 days ended with Tony Blair's first Commons defeat as prime minister in 2005. Instead it was extended to 28 days.
The issue was reopened at the weekend when the head of the Association of Chief Police Officers, Ken Jones, said police needed more flexibility and suspects should be held "for as long as it takes".
But speaking at a Westminster press gallery lunch Mr Davis said any attempt to extend the period to 90 days could alienate Muslims.
He said Parliament made the right move in 2005 by extending the period to 28 days, but the government would be wrong to now demand an extension to 90 days.
Instead he said powers could be granted if the government declared a national emergency, which would allow it to hold suspects for a further 30 days, on top of the current 28-day limit.
Ministers have said they want a political consensus on the anti-terrorism laws.
Mr Davis told the BBC an agreement between the main parties on a new counter terrorism bill - covering issues like the use of intercept evidence in court - was not yet close.
But he said he felt a deal was likely before the bill was presented to Parliament in October.
On Monday Security Minister Lord West and the government's independent reviewer of terrorism laws, Lord Carlile, said they supported Mr Jones's calls for the 28-day limit to be extended.
Lord Carlile suggested that senior judges, not politicians, should decide how long someone is held.
Police have argued that the increased complexity of terrorism plots means they may need longer to interview suspects before they are charged.
Once they are charged, they cannot be questioned - although that might be reviewed in the counter terrorism bill.
On Monday Lord West said there had been no cases which had run over 28 days, but said one alleged plot had gone "right up to the wire".
Police had to have the time to gather evidence for conviction, he said, as terrorists were getting "cleverer and cleverer" at hiding data.
"This is a real threat to this nation and we've got to somehow confront it," he told the BBC.