Police are currently tracking a number of suspected terror plots
The home secretary has described the terrorism threat facing the UK as "severe" and "growing".
In a News of the World interview, Jacqui Smith said 30 terror plots were being investigated and police needed to be able to detain suspects for longer.
But one Labour MP said he hoped the government would be defeated over its plans to extend the time suspects can be held without charge to 42 days.
The Tories and LibDems say increasing time limits will not help fight terror.
Ms Smith said: "We now face a threat level that is severe. It's not getting any less, it's actually growing.
"There are 2,000 individuals they are monitoring. There are 200 networks. There are 30 active plots.
"That has increased over the past two years. Since the beginning of 2007, 57 people have been convicted on terrorist plots.
"Nearly half of those pleaded guilty so this is not some figment of the imagination. It is a real risk and a real issue we need to respond to.
"We can't wait for an attack to succeed and then rush in new powers. We've got to stay ahead."
Under the new proposals the home secretary would be able to immediately extend the detention limit of a suspect to 42 days, as long as it was supported by a joint report by a chief constable and the director of public prosecutions.
The extension would then have to be approved by the Commons and the Lords within 30 days. But if either House voted against it, the power would end at midnight on the day of the debate.
The government faces a rebellion within its own ranks over the plans - in 2005 it was defeated over proposals to extend the limit to 90 days.
Labour MP Frank Cook told the BBC the current proposals would be a "recruiting agent for terrorism".
Mr Cook, MP for Stockton North, did not vote when Tony Blair tried to introduce the 90 day limit in November 2005 for Parliamentary reasons, but said that at the time he was "minded to support the government".
He said: "At the time I didn't feel it was a problem. Only later I realised there were not enough safeguards," he said.
Last week former cabinet minister Frank Dobson warned that opposition by Labour backbenchers has increased, because no evidence had been produced of a need to go beyond the present 28-day limit.
The proposals are supported by some senior police officers but, if passed, could face a court challenge from the Equality and Human Rights Commission.
Last November, Jonathan Evans, MI5's director general, spoke of 2,000 people posing a threat to the UK - the same number quoted by the home secretary.
However, he went on to say that the number had not peaked.
In November 2006, the then MI5 director general, Dame Eliza Manningham-Buller, again spoke of 30 different terror plots and said 1,600 individuals were being monitored.
Tony Blair's former attorney-general, Lord Goldsmith, again stressed his opposition to the 42-day limit.
He told Sky News that Ms Smith and the prime minister were embarked on a course that was wrong
"I still hope that they may see that ultimately it doesn't make sense to pursue this, and they will get huge support from a lot of other people - from me in particular - for all the other steps that they want to take in order that we have a tough, firm but fair policy on prevention of terrorism."
Conservative security spokeswoman, Dame Pauline Neville-Jones, told the BBC she did not agree with the government's approach.
"If there is an increase in plots it doesn't follow, I fear, that the right way to deal with that is actually then to hold suspects for an even longer time.
"What you need if you've got an increase in plots is the right quantum of resources for both the police and the intelligence services to track and disrupt the plots - and that's a question of bringing resources to bear."
Lib Dem home affairs spokesman Chris Huhne said the move to extend the time limit for holding terror suspects was "ridiculous".
"We have a 92% conviction rate on terrorist offences. For heavens sake that's far higher than on ordinary criminal cases.
"So what on earth is the government on about in saying that we need to go further than any comparable country has anywhere else in the world?
"In Canada it's still one day, and we're talking about 42. In America it's only two days and we're talking about 42. In Australia it's 12 days."