Gordon Brown last night sought to calm fears over threats to traditional civil liberties which were raised after he signalled a fresh counter-terrorism crackdown to combat "a new security problem" facing Britain.

Speaking at Labour hustings in Newcastle-upon-Tyne, the Chancellor, who takes over as Prime Minister on June 27, reassured his party's grassroots that he would introduce safeguards to protect civil liberties as he flagged up his intention to revive measures which would allow the police to hold suspects for more than 28 days without charge and be able to question a detainee after he or she had been charged.

He is also planning to introduce new laws making terrorism an aggravating factor in sentencing in a similar way to existing laws on racially aggravated crime.

The proposals - some of which are due to be outlined by Home Secretary John Reid this week - sparked concern among civil liberty campaigners and MPs, who warned that extending pre-charge detention to 90 days would amount to internment.

In response, the in-coming PM sought to reassure them by proposing a requirement for a weekly judicial review of a detained suspect and establishing cross-party talks on the possible use of phone-tap evidence in court.

In Glasgow over the weekend, Mr Brown sought to put the terrorist threat into context by saying that al Qaeda "probably" had as many as 60 offshoots around the world, using increasingly sophisticated means.

Yesterday, he declared how he was ready to be "tough in the security measures that are necessary to prevent terrorist incidents in this country," noting: "We will have to consider further legislation to do so. That is where the public will need to recognise that we have a new security problem."

However, he said he would make sure that "at no point will our British traditions of supporting and defending civil liberties be put at risk".

He added: "There has got to be independent judicial oversight. There has got to be parliamentary accountability. We should give the police the power to question people so we can both prevent incidents and get to the bottom of some of these very strange dealings."

Previous plans to extend pre-charge questioning to 90 days were scuppered by a Labour back-bench rebellion in 2005 when Tony Blair suffered his first Commons defeat as PM.

Yesterday, senior Labour MPs, including deputy leadership candidate Harriet Harman, suggested Mr Brown would need to present compelling evidence that the current 28-day limit was putting the public at danger before he could convince them to allow the measure through this time.

"If the government is going to bring forward and propose the powers that we say the police need, then we have to produce the evidence of why the current law is inadequate and we have to show the safeguards that will be there for civil liberties. That is the way to achieve consensus," argued Ms Harman.

Fellow deputy leadership contender Peter Hain, the Northern Ireland and Wales Secretary, agreed tougher powers were needed but also made clear there had to be a "careful balance between individual liberty and security".

David Davis for the Conservatives noted how Mr Brown had publicised his plans just days before Mr Reid's statement to MPs.

"It does not augur well for cross-party attempts to build a consensus for the new counter-terrorism measures, which the whole country needs to get behind," insisted the Shadow Home Secretary.

Nick Clegg, the Liberal Democrats' home affairs spokesman, noted how there was not much fresh thinking in Mr Brown's plans but accepted that he at least appeared to be "a little more concerned about parliamentary accountability than his predecessor".

He added: "It now remains to be seen whether this is just a procedural fig-leaf for more authoritarian measures or part of a genuine shift in guaranteeing and not undermining fundamental civil liberties."

Liberty, the civil rights group, said it supported the use of phone-tap evidence and post-charge questioning as alternatives to extending detention before charge but not in addition to it.

Shami Chakrabarti, its director, said Mr Brown was making "a grave mistake" in proposing to extend questioning without charge beyond 28 days, which, she pointed out, was the longest such period in the free world. "If you go beyond 28 days, it is internment," she added.

Meantime, Elish Angiolini, Scotland's Lord Advocate, said the changes being suggested for anti-terror laws would be applied in Scotland but she made clear she hoped to ensure they would be compliant with Scots law.

She also noted that any proposed new law would have to be compliant with human rights legislation, pointing out Scottish courts placed a deadline on the time it took for a case to come to court while they required a higher standard of corroborated evidence in criminal cases.