Among other things, the government opposes attempts by Khadr's lawyers to now allege that Canada conspired with the United States to breach his rights, documents obtained by The Canadian Press show.
"The proposed amended statement of claim...reads like a colourful piece of investigative journalism rather than a pleading in a legal proceeding," the government argues.
"(It) is a lengthy polemic consisting largely of argument."
Khadr's lawyers originally filed a $100,000 lawsuit in March 2004 alleging the federal government had breached his rights when security agents went down to interview him at the U.S. naval base.
They amended the claim in 2009, when Khadr sought damages of $10 million, to argue in part that Canadian officials knew the U.S. had tortured him by depriving him of sleep at Guantanamo Bay.
The new amended lawsuit, proposed in the summer, now seeks at least $20 million in damages.
Among other things, it alleges Canada's negligence in conspiring with the United States in the "arbitrary detention, torture, cruel, inhuman and degrading treatment" of the Toronto-born Khadr.
"Canada is a joint conspirator with the United States in ensuring the long-term detention of Omar Khadr," John Phillips, one of Khadr's Canadian lawyers, said in an interview Tuesday.
"They are equally complicit."
Amended claims in Federal Court are generally routine but the government argues vigorously against allowing Khadr, 27, to change his claim.
"The plaintiff is attempting to do indirectly that which he is not allowed to do directly: that is bring a claim against the United States government in a Canadian court," the feds say in filings ahead of a court hearing this month.
In any event, the government maintains, Federal Court has no jurisdiction to judge American conduct or the legality of Guantanamo Bay.
American soldiers arrested the badly wounded Khadr, then 15 years old, following a fierce four-hour battle in Afghanistan in July 2002 in which a U.S. special forces soldier was killed.
Khadr was held captive in Guantanamo Bay, ultimately pleading guilty to five war crimes before a widely condemned military commission in October 2010 in exchange for a further eight-year sentence. He was transferred to Canada in September 2012 to serve out his punishment, and is currently held in an Edmonton penitentiary.
Khadr's lawyers argue the plea was "coerced" given that he could have faced indefinite detention even if acquitted.
"That plea agreement has no force or effect and should be given no force or effect in Canadian law," Phillips said.
In separate filings, his lawyers are challenging both his underlying conviction and his incarceration as a maximum-security inmate.
The government, which has consistently branded Khadr a dangerous terrorist, has previously conceded knowing about the sleep deprivation when Canadian security agents grilled the prisoner.
The Supreme Court has also found the government violated Khadr's rights.
The government argues Khadr's new proposed claim is too different from the original to count as an amendment, and lacks the specifics that would allow for a focused defence, according to the court documents.
At most, the government says, Federal Court should allow only minor changes.