The UK's most senior police chief said there was "chilling" evidence of new terrorist plots as he urged support for new powers to detain terror suspects.
Sir Ian denies entering the political arena
Metropolitan Police chief Sir Ian Blair said police should be able to hold suspects without charge for 90 days.
His warning came as the government said it was increasingly confident it would win an MPs' vote on the new time limit.
But the Tories, Lib Dems and some Labour MPs say 28 days' detention is the maximum they will accept.
Government officials admit the numbers ahead of the vote on Wednesday are very tight.
The proposal to extend the detention time limit from 14 to 90 days originally came from the police and Sir Ian used a lunch with political journalists at Westminster to reinforce their case.
He said the police were "not in an auction" over the plan but believed 90 days was the right length of time needed because of the complexity and mass of evidence in terrorism cases.
"I do accept what we are putting forward is unknown in peacetime," he said.
"But I have never seen anything like what's happening at the moment. There are people out there in the UK plotting mass murder without warning."
He said he had thought hard about whether the police had entered a political debate.
But he said: "This is not in my view politics. This is our professional opinion."
Sir Ian said 90 days was not a "magic" figure and four months might be better.
But Tory leader Michael Howard said: "The logic of what they are saying is that there shouldn't be any limit at all and that we should have quite unlimited detention without charge."
That was unacceptable, he added.
Shadow attorney general Dominic Grieve said he was sorry Sir Ian had intervened in a political debate.
"Being brought in for the specific political purpose of beefing up the government's position seems to me to be unacceptable," he said.
Alistair Carmichael, a Liberal Democrats home affairs spokesman, was also critical of the police getting involved.
"On such a controversial matter and at such a politically sensitive time, they need to tread very carefully when intervening in public," he said.
"Because the government is no longer trusted, they are all too keen to see the police take a lead in this debate."
Last week, Home Secretary Charles Clarke promised to hold talks on the issue when the 90-day proposal faced a possible Commons defeat.
But ministers are now sticking by their original plan, although they also have a "fall back" position of 60 days.
In a last ditch attempt to persuade Labour MPs to back the 90-day plan, Mr Clarke reminded them in a mass email on Tuesday evening that he had attached a "sunset clause" to the measures which means they will have to be reviewed after 12 months.
He said a High Court judge had to agree an extension of detention every seven days and a code of practice would govern the treatment of those being held.
He told MPs: "These are all significant changes from the original government proposals and I hope that they will mean that MPs who previously had doubts will feel able to support the government."
Tony Blair's official spokesman said on Tuesday: "There is a growing acceptance for 90 days but the government takes absolutely nothing for granted."
And Chancellor Gordon Brown backed the plan in his first public comments on the issue.
Mr Brown said he was "shocked and surprised by the short-term opportunism" of the Conservatives.
He accused leadership contenders David Cameron and David Davis of failing to put the advice from the security services first.
Mr Davis, the shadow home secretary, called that suggestion "laughable".
"What we're arguing is this is a matter of high principle - that hundreds of thousands, if not millions, of people died to defend the right to presumption of innocence, the right not to be locked up without trial."
Some Conservative MPs, including former minister Ann Widdecombe, say they will vote for the government's 90-day proposal.
But Lib Dem leader Charles Kennedy predicted ministers would not get the 90-day plan through Parliament.
"It is reprehensible and regrettable that the prime minister is just throwing consensus out of the window - or indeed any genuine efforts to achieve consensus," said Mr Kennedy.
MPs will on Wednesday vote on an amendment, introduced by Labour backbencher David Winnick, calling for the 28-day compromise.