The supreme court is to hear arguments this week on whether security certificates that allow secret court hearings, undisclosed evidence and open-ended detention are constitutional.
The certificates were introduced under the immigration act in 1976, but since September 2001 they have been used to jail five Muslims - Adil Charkaoui, Mohamed Harkat, Hasan Almrei, Mahmoud Jaballah and Mohammed Mahjoub - suspected of links to terrorists abroad or membership of a terrorist group.
Canadian MPs say the measures are needed to thwart terrorist attacks, but lawyers for three of the detainees say they breach civil liberties.
Federal lawyers argue that secrecy prevents disclosure of intelligence and surveillance techniques to terrorists abroad.
They said in a court brief: "The greater the number of individuals with access to this information, the greater the risk of advertent and inadvertent disclosure. What is fair depends entirely on the context."
Civil libertarians hope that the court will undo what they call a hysterical reaction to security threats.
"Civil liberties are basic to every person... We can't permit this idea of pre-emptive detention"
James Loney, former hostage
Joe Comartin, an opposition MP, said the certificates "are anathema to Canadian criminal law values and our Charter of Rights and Freedoms".
James Loney, a former Canadian hostage in Iraq, said: "They're being held on suspicion of what they might do at some point in the future. Civil liberties are basic to every person... We can't permit this idea of pre-emptive detention."
Critics also question the quality of Canada's intelligence in such cases, much of it obtained from other countries, and whether racial profiling was involved in the arrests. A ruling is expected to take about six months.
The case comes after 17 Muslim males, five of them under 18 years old and all either Canadian citizens or residents, were detained as terrorist suspects in Toronto on June 2. They are being charged under an anti-terrorism act.
Although this is newer than the immigration act, the supreme court's decision is likely to set a precedent in how all terrorist suspects are treated.
Defence lawyers said on Monday that a publication ban on the court hearings that precede the formal trial is another sign that their clients have no chance of justice.
Rocco Galati, one of the defence team, said of the prosecuting authorities: "After they've had 10 days with the media, feeding the media with whatever they want to feed the media ... they now have the audacity to request a publication ban of all proceedings from today's date."
Parliament is reviewing its anti-terrorism measures at the moment and Galati condemned what he described as "a show trial for political ends".
He said that the intention was "to influence the vote in the House of Commons on extending the anti-terrorism provision and to influence the supreme court ... in its constitutional review of anti-terrorism provisions".