Liberal Democrats will not give the government a "blank cheque" over its anti-terror legislation, the party's home affairs spokesman has said.
Lib Dems are unhappy with plans to extend detention without charge
Mark Oaten said plans to extend custody without charge to a maximum of 90 days and making "glorifying" terror an offence would damage civil liberties.
He said the Lib Dems would vote against these plans in Parliament - breaking the cross-party anti-terror consensus.
Ministers say complex terror inquiries create a need for longer custody.
The UK should not "abandon" the "right to a fair trial" in the wake of the July bombings in London, Mr Oaten told the Lib Dem annual conference in Blackpool.
He said: "If we give up the fundamental principles of justice, we are giving in to the terrorists.
"And if we sacrifice our liberal society we will be weaker, not stronger."
He said he would work with Home Secretary Charles Clarke, adding: "But we will also defend the rights and liberties of the country."
Speaking earlier at a press conference, Mr Oaten said there had been a "genuine consensus" on terror legislation in the wake of the London bombings, when the opposition parties were "discussing a limited number of measures" with Mr Clarke.
But Tony Blair's announcement of a raft of additional proposals at the beginning of August, including banning people who advocate terror, "was extremely unhelpful" and led to that consensus being broken.
Former parliamentary party chairman Matthew Taylor says the Lib Dems should not be even talking to the government about plans to extend the time limit for holding terror suspects without trial to three months.
He says the current 14-day limit is too long.
Mr Oaten also called in his speech for an end to the "national disgrace" of "crumbling" Victorian-built prisons.
Within two years of release, 59% of prisoners were back in court, he added.
The government's anti-social behaviour orders (Asbos) would receive only "limited" support from Liberal Democrats, Mr Oaten said.
"The problems of anti-social behaviour are not going to be solved by focusing simply on the symptoms and not the causes.
"That's why our support for Asbos is limited, as they are a quick-fix solution."
The party needed to find ways of "knitting communities together", Mr Oaten added.
Earlier, party leader Charles Kennedy refused to say whether he would agree to any extension in the detention time limit.
"We are going to take it one step at a time but we are quite clear that this degree of extension cannot be acceptable," he said.
Three months' detention without trial would be tantamount to suffering a six-month jail sentence, argued Mr Kennedy.
Police chiefs say the complexity of terror cases means they need longer to conduct forensic tests and sift through warehouses full of evidence.
The Lib Dems also oppose plans for a new crime of "glorifying" terrorism.
They say it is adequately covered by a separate offence of indirect incitement, which will be included in the anti-terror legislation.
But the party backs moves to outlaw indirect incitement of terrorism, training of terrorists and "acts preparatory to terrorism".
Mr Clarke should instead give more resources to the Serious Organised Crime Agency to allow greater support for county forces, Mr Oaten said.
The Leader of the House of Commons, Geoff Hoon, said the government was trying to get consensus on the right balance between liberty and security.
He told ITV News Channel he hoped the Lib Dems would reflect on the London bombings and "recognise that we do not put forward these proposals lightly".