"I am a terrorist," Omar Khadr allegedly told U.S.

In this courtroom sketch, Canadian defendant Omar Khadr (L) attends the first day of his hearing in the courthouse for the U.S. military war crimes commission at the Camp Justice compound on Guantanamo Bay U.S. Naval Base in Cuba August 9, 2010.

GUANTANAMO BAY U.S. NAVAL BASE, Cuba (Reuters) - Canadian prisoner Omar Khadr told interrogators he was an al Qaeda terrorist and described pulling the pin of a grenade that killed a U.S. soldier in Afghanistan, a prosecutor told Khadr's war crimes tribunal on Thursday.

But Khadr's defense attorney said those were the words of a scared and wounded child whose interrogators frightened him into giving a false confession by making up a tale of a young boy gang-raped and killed in prison.

"It is only after that story is told to Omar Khadr that he admits to throwing anything. He told them what they wanted to hear," Army Lieutenant Colonel Jon Jackson said in defense opening statements.

The first day of testimony ended early and dramatically when Jackson fainted in the courtroom. He was questioning a witness, asked for a recess and fell to the floor with a thud.

Jackson suffered complications related to recent gall bladder surgery and was in the base hospital, Deputy Chief Defense Counsel Bryan Broyles said on Thursday night. Jackson is Khadr's only lawyer and the trial cannot resume until he is medically cleared, Broyles said.

Toronto-born Khadr was 15 when captured during a firefight at an al Qaeda compound in Afghanistan in 2002. He is the first person since World War Two to face trial in a military tribunal for acts allegedly committed as a minor.

The United Nations said earlier this week that the trial at the Guantanamo Bay naval base was of dubious legality and could set a dangerous precedent for child soldiers worldwide.

Now 23, Khadr is accused of killing 1st Sergeant Christopher Speer with a grenade during the battle and making roadside bombs to target U.S. troops.

Prosecutor Jeff Groharing said Khadr was raised in a family of Islamist extremists who spent holidays with Osama bin Laden, trained their boy to use bombs and guns and encouraged him to kill Americans.

"I am a terrorist trained by al Qaeda -- those are Omar Khadr's own words," Groharing told the seven military officers on the jury. "Those words were confirmed by his acts."

He said Khadr described in detail pulling the pin of a grenade and lobbing it over his shoulder at U.S. special forces who entered the mud-walled compound.

The grenade exploded at Speer's feet and hurled shrapnel into his brain.

Jackson said Khadr was in the compound with three "bad men" and that one of them threw the grenade that killed Speer before being fatally shot himself by the U.S. troops.

"Omar Khadr did not kill Sergeant Speer. He has been waiting eight long years to tell you that. To tell somebody who can finally listen and who can finally make a difference," Jackson told the jury.

Khadr was in the compound only because his father, Ahmed Khadr, took him there to translate for the bomb-makers, he said. "He was there because Ahmed Khadr hated his enemies more than he loved his son," Jackson said.

U.S. soldiers described the four-hour battle that began when they sent two Afghan militiamen to the compound to ask the occupants to come out. The Afghans were shot and killed and U.S. forces responded by dropping two 500-pound (225-kg) bombs on the compound and unleashing a torrent of fire from planes and helicopters.

A special operations soldier identified only as Sergeant Major D testified that when the bombing ended, he went into an alley inside the compound and saw two people still alive -- an adult man lying alongside an AK-47 rifle and someone sitting behind a pile of brush facing away from him.

D said he shot the first man in the head and killed him, then shot the seated person, Khadr, twice in the back. Khadr was blinded in one eye by shrapnel and had gaping exit wounds on his chest.

U.S. medics patched him up and took him to a field hospital in Bagram, where he was treated and interrogated until being sent to Guantanamo just after his 16th birthday.

The commander at the battle, identified only as Colonel W, wrote in his initial report that the enemy who threw the grenade had been "killed." When prosecutors interviewed him two or three years later to prepare charges against Khadr, D gave them a copy that said the grenade-thrower had been "engaged."

D testified that he originally thought Khadr had been fatally wounded but altered the report later to show he survived. Jackson contends the man with the AK 47 threw the grenade and the report was altered later to implicate Khadr.

(Editing by Eric Walsh)