Mr Brown is facing a potential fight from some backbenchers
Gordon Brown has written to all Labour MPs, seeking their support for plans to extend the time terror suspects can be held without charge to 42 days.
The Commons vote on extending the limit from 28 days is on Wednesday.
The prime minister has told MPs that new safeguards mean suspects would only be held for six-weeks during "a terrorist crisis situation".
The government faces a battle to get the proposals through Parliament, amid claims they infringe civil liberties.
More than 30 Labour backbenchers are said to be ready to defy ministers in a crucial Commons vote on the issue
The Conservatives and Liberal Democrats also oppose the plans.
This raises the prospect of Mr Brown suffering his first Commons defeat since becoming prime minister last year.
But ministers are hoping the fears of potential rebels have been assuaged by an address to the Parliamentary Labour Party by Home Secretary Jacqui Smith.
In that, she proposed new safeguards to prevent arbitrary use of the new powers.
However, other critics of the plans include the former Prime Minister, Sir John Major, who told the BBC's Today programme: "I look at what is proposed and I have a substantial amount of doubt as whether it will actually be successful.
"Nobody has yet been able to explain how the various safeguards are going to operate - how parliament interacts, how the independent advisor will work, precisely what the role of the DPP will be and what happens if the three advisors to the home secretary take a contrary view."
Mr Brown's letter contains no new concessions but says ministers have done "everything in our power" to balance liberty with the need for enhanced security against "individuals and groups who are prepared to use suicide attacks and want to cause mass casualties without warning".
"In the legislation currently before Parliament, we have done everything in our power to protect the civil liberties of the individual against any arbitrary treatment, because in Britain liberty is, and remains, at the centre of our constitutional settlement," Mr Brown told his party colleagues.
"The challenge has been to make sure that, through proper judicial and parliamentary oversight, we both keep the public free from the threat to our security, and secure the fundamental liberties of the citizen."
He also rebuffed the argument that 42-day pre-charge detention will put the UK outside the mainstream of Europe.
Mr Brown also said he was confident that what was proposed fully complied with the European Convention on Human Rights.
He also restated his argument that the terrorist threat today is "radically different" from that posed by terror organisations in the past.
The prime minister said al-Qaeda supporters had the means to create multiple identities, complex computer networks, and operations over many countries.
Mr Brown, who has a working majority of 65, has made clear that he does not regard Wednesday's vote as a matter of confidence in his premiership.
Senior police officers have backed the 42-day plan, but the director of public prosecutions Sir Ken Macdonald said that Ms Smith's safeguards had made no difference to his belief that it was unnecessary.
Parliament's Joint Committee on Human Rights dismissed the new safeguards as "inadequate" and said the case had not been made for justifying the longer period.