By David Ljunggren

OTTAWA, Sept 19 (Reuters) - The head of Canada's main opposition party demanded on Wednesday that the government press the United States to try a young Canadian man who has been imprisoned at Guantanamo Bay for five years.

Omar Khadr, 21, is accused of killing one U.S. soldier with a grenade and wounding another during a firefight at a suspected al Qaeda compound in Afghanistan in 2002. He is the only Canadian at Guantanamo.

Rights groups say the jail should be shut on the grounds that holding prisoners for years without trial violates international standards.

They also say Khadr should be treated as a child soldier, since he was 15 at the time of alleged killing.

"The government of Canada and the prime minister himself must demand that Omar Khadr be removed from the Guantanamo Bay detention facility and transferred to the United States to be tried in a legitimate court," said Liberal leader Stephane Dion.

"Mr Khadr must also be afforded legal protections and guarantees that we would insist on any other Canadian citizen receiving ... and he cannot face the possibility of the death penalty," he told reporters in Toronto.

The chances of the minority Conservative government listening to Dion seem remote, especially since Prime Minister Stephen Harper strongly supports the U.S.-led war on terror. Canada has 2,500 troops in Afghanistan.

In June, a U.S. judge for a special war crimes tribunal set up for Guantanamo cases dismissed murder and conspiracy charges against Khadr but military prosecutors appealed the decision.

Canada's Foreign Ministry said Khadr still faced serious charges.

"Any questions regarding whether Canada plans to ask for the release of Omar Khadr from Guantanamo are premature and speculative," said a ministry spokeswoman.

Dion spoke after a meeting with Lt-Commander William Kuebler, a military lawyer representing Khadr. Kuebler, who backed Dion's call for action, described the special tribunals as a mockery of justice.

"He (Khadr) is going to face trial in a system that was created entirely after the fact to produce convictions in a known set of cases," he told the Canadian Broadcasting Corp, noting that Australia and Britain had intervened in the cases of their citizens in Guantanamo.

"It's ridiculous to say at this point that the Canadian government can be content with assurances from the United States that Omar is being treated fairly," Kuebler said.

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