The number of terrorist plots in the UK is "mounting" and the "magnitude" of their ambitions growing, Met Police chief Sir Ian Blair has warned MPs.
Sir Ian Blair wants extension without charge to be extended
He was making the case to the Commons Home Affairs committee for extending the current 28-day limit for detaining terror suspects without charge.
Terror law watchdog Lord Carlile told MPs longer detention would not harm relations between Muslims and police.
He said foreign policy was more likely to "radicalise young people".
The government has said the time has come to re-examine the 28-day limit - which was doubled from 14 days in 2005 - because of the complexity and nature of the threat of terrorism.
Its previous attempts to extend the period to 90 days ended in defeat - when it was rejected by a combination of Tory, Lib Dem and some Labour MPs.
Sir Ian conceded that there have been no cases so far where detention beyond 28 days had been needed but he said it was only a matter of time.
He told the home affairs committee internment was a bad idea - but 50 to 90 day detention would be "sensible" if there was judicial oversight.
"The prospect that we will need more than 28 days some time in the not too distant future is so real a prospect that Parliament needs to consider it," Sir Ian told MPs.
The Metropolitan Police chief conceded that extending detention beyond 28 days would not "ease" relations between police and the Muslim community.
But he said more effort should be made to explain to the Muslim community why the police believed it was needed.
"The number of the conspiracies, the number of conspirators within those conspiracies and the magnitude of the ambition, in terms of destruction and loss of life, is mounting, has continued to mount year by year," he said.
Sir Ian added that given that increase in threat "a pragmatic inference can be drawn that at some stage 28 days is not going to be sufficient".
But both the Conservatives and Liberal Democrats said there were many alternatives to extending the detention period, including allowing post-charge questioning and intercept evidence in trials.
Shadow home secretary David Davis said: "We have the longest period of detention without trial in the democratic world.
"Any increase needs to be based on evidence - not guesswork - that it is needed to protect the public."
And Lib Dem home affairs spokesman Nick Clegg said: "In the absence of any compelling evidence it is a mystery why Sir Ian Blair, Gordon Brown and [home secretary] Jacqui Smith seem so determined to reopen this debate."
The government's independent reviewer of terrorism laws, Lord Carlile, agreed there was no evidence of a case so far in which it could be shown that detention beyond 28 days would have made a significant difference.
But he said in a very small number of cases - perhaps two or three in the next five years - more time for police questioning could be useful, and in his view would be justified, subject to oversight by judges.
He told MPs he said any number of days would be "entirely arbitrary" and there was no "perfect cut-off point".
"I've recognised that Parliament will set a cut-off point. My view is that probably, even if there was a limit of 90 days, almost nobody, perhaps nobody at all, would be held for 90 days if there was a proper and fully human-rights compliant, system of judicial control."
Lord Carlile told MPs he did not think extending the pre-charge detention limit would damage relations between Muslims and the police.
He said he was concerned that Muslims were disproportionately affected by detention laws as the "vast majority" of Muslims disapproved very strongly of terrorism.
"But I don't believe that from that one can necessarily extrapolate: 'Ergo - the extension of detention from say 28 to 56 days is going to make relations with the police worse'. I see no evidence of that, none has been given to me and I don't think it's logical.
"There are other things that radicalise young people of course, like foreign policy for example, but that's a different matter."