Prime Minister Tony Blair has insisted there is a "compelling" case for the anti-terror laws proposed in the newly published Terrorism Bill.
Ministers want to hold terror suspects for up to 90 days
The bill allows suspects to be held for up 90 days without charge and makes "glorifying" terror an offence.
At prime minister's questions, Charles Kennedy said there was no "consensus" even within the government on measures, some of which were plain "wrong".
But Mr Blair said he believed police needed the powers to protect lives.
'Charge them properly'
The prime minister said the complex nature of terrorism inquiries meant it was important to take suspects into custody "relatively early", and possibly holding them for longer.
Police could then "get the evidence necessary to charge them properly".
He added: "What I have to do is to try do my best to protect people in this country and to make sure their safety and their civil liberty to life come first."
Currently terror suspects can be held for up to 14 days without charge.
The government's plans have been criticised by senior judges and opposition parties.
The Lib Dems have suggested holding suspects on lesser offences while investigating possible terrorism, as an alternative to extending detention without trial.
Mr Kennedy said other measures in the bill - such as a charge of committing acts "preparatory to terrorism" - could be used for this.
He said: "Why is it you remain so wedded to this proposal for 90 days? Surely it's wrong; surely you are going to have to back down."
Conservative home affairs spokesman David Davis said that he had been briefed by the police on their need for the new powers, but had not been persuaded.
He said that he would prefer a change in the law so that suspects could still be questioned even after being charged with an offence.
And John McDonnell, chairman of the Campaign Group of left-wing Labour MPs, said: "This is an unacceptable undermining of civil liberties in the New Labour tradition of knee-jerk legislation."
BBC political editor Nick Robinson says concessions are likely as ministers try to win over Labour backbenchers to support the bill.
Liberal Democrat peer Lord Carlile, appointed by the government to oversee its anti-terrorism legislation, said he still had "concerns".
He wanted a "much tighter system of control", to ensure longer detentions were "justified...by evidence".
Ken Jones, chairman of Association of Chief Police Officers' terrorism committee, said longer detentions were needed because of "the huge volume of material we are discovering and the complex global reach of our inquiries".
A group including London Mayor Ken Livingstone and Muslim leader Sir Iqbal Sacranie later met at Westminster City Hall to protest against the bill.
Sir Iqbal, secretary general of the Muslim Council of Britain, said: "We all need to be vigilant in ensuring that the government's proposed measures do not jettison fundamental freedoms at the cost of providing little or no guarantee of extra security."
Foreign Secretary Jack Straw later published a dossier detailing how 10 other western countries dealt with terror suspects.
But Lib Dem home affairs spokesman Mark Oaten said the document undermined ministers' arguments.
Foreign Office officials acknowledged that none of the other countries had powers to detain suspects for up to three months without charge.
Officials said the UK could not be directly compared with the other countries because - unlike elsewhere - the police could no longer question a suspect once they had been charged.