By David Ljunggren

OTTAWA, Aug 8 (Reuters) - Lawyers for a young Canadian man imprisoned at Guantanamo Bay filed a lawsuit on Friday against Prime Minister Stephen Harper in a bid to force him to intercede with Washington on the inmate's behalf.

Harper has so far refused to ask the United States to repatriate 21-year-old Omar Khadr, who is due to go on trial in October on charges of killing a U.S. soldier in Afghanistan in July 2002, when he was 15.

Critics say Khadr was a child soldier and should be helped rather than punished. Harper, whose right-wing Conservatives won power in January 2006 on a law-and-order platform, says the man is facing serious charges.

The suit wants Canada's Federal Court to order Harper to intervene before the U.S. military trial starts.

"We're doing it to compel Stephen Harper to finally do the right thing and stand up for the rights of a Canadian citizen," said Lieutenant William Kuebler, Khadr's U.S. military lawyer.

"If a Canadian court directs him to do it I don't think he can say 'Get lost'," Kuebler told Reuters by telephone.

Last month, video footage of Canadian agents interrogating Khadr at the U.S. prison at Guantanamo Bay, Cuba, in 2003 showed the then teenager weeping and calling for his mother.

Khadr -- the only citizen from a western nation still in the jail -- has alleged U.S. interrogators repeatedly threatened to rape him.

Harper says Ottawa is pressing Washington to ensure Khadr is treated humanely. Documents released last month show U.S. authorities deprived Khadr of sleep ahead of a separate interview with an official from Canada's Foreign Ministry in 2004 and informed the Canadians what they had done.

"The Canadian government knew about, facilitated and indeed helped cover up the torture and abuse of a Canadian citizen and I think that ... takes us from a question of policy into a question of legal obligation," Kuebler told Reuters.

A statement from Khadr's lawyers said that under a United Nations protocol designed to help child soldiers, "Omar is entitled to special protections under international law, including opportunities for rehabilitation and social re-integration".

Harper's press office did not return calls or e-mails seeking comment.

Khadr was taken to Afghanistan by his father Ahmed Said Khadr, an alleged al Qaeda financier and close friend of Osama bin Laden. Khadr senior was killed in a battle with Pakistani forces in 2003. (Reporting by David Ljunggren; editing by Rob Wilson)

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