Omar Khadr's pre-trial hearing began in Guantanamo Bay Wednesday afternoon, hours after his lawyers turned down a purported deal that would have seen their client serve five years in a U.S. prison as a trade-off for pleading guilty.

The 23-year-old Khadr has been in U.S. custody since he was 15 years old, after being captured in Afghanistan and accused of throwing a grenade that killed a U.S. soldier.

The case involving Khadr -- a Canadian citizen and son of Ahmed Said Khadr who died in a 2003 battle -- will serve as a test run for the U.S. military justice system, which is supposed to process the remaining Guantanamo detainees so that Washington can shut down the controversial prison.

In a case that has taken many twists and turns over the years, Khadr is now at a pre-trial stage, in which his lawyers are trying to have key evidence -- a video that appears to show a young Khadr learning how to build an IED -- thrown out on the grounds that it was not obtained properly.

Under the U.S. Military Commissions Act of 2009, evidence that is obtained through cruel, inhumane or degrading treatment of a prisoner is not admissible.

Khadr's lawyers say that under these rules, the video should not be allowed as evidence.

He appeared in a military courtroom on Wednesday morning, but his hearing was pushed back to the afternoon after the prosecution and defence teams acknowledged they needed more time to review the rules for the military court.

CTV's Lisa LaFlamme said they did not receive copies of the 281-page set of rules until late Tuesday evening.

"Strange as it sounds, on the eve of these very significant -- crucial, in fact -- pre-trial hearings, there was, in fact, no rulebook for the defence to even operate under," LaFlamme told Â鶹´«Ã½ Channel from Guantanamo Bay on Wednesday morning.

The timing of the rulebook's release was unprecedented, according Denny LeBoeuf, a lawyer with the American Civil Liberties Union. It called into question the fairness of the process, she said.

Such rules are usually released months before they come into effect so that lawyers have ample time to study them.

"There is no set of circumstances like this," LeBoeuf told The Canadian Press from New Orleans.

"This would literally never happen, not in the most be-knighted backwoods court you can imagine. It would simply never happen."

The hearing began after Khadr's lawyers apparently rejected a plea deal from U.S. prosecutors, LaFlamme said earlier on Wednesday.

"Prosecutors have offered a sentence of five years -- either here in Guantanamo Bay if it is not closed, or in a U.S. prison -- if Khadr pleaded guilty to war crimes offences."

"The defence turned down that offer. They had been talking, in fact the defence openly discussed yesterday the fact that plea bargain negotiations were ongoing."

The fact that Khadr received a plea offer may suggest that the prosecution isn't confident it could get a conviction on the murder, LaFlamme said.

With files from The Canadian Press