Salim Hamdan, a Yemeni, has been in custody for nearly six years
A former driver of Osama Bin Laden has pleaded not guilty at the first war crimes trial to be held in the US prison in Guantanamo Bay.
Yemeni national Salim Hamdan, 37, is accused of conspiracy and supporting terrorism, and faces life in prison if he is convicted.
The right of the military tribunal to try him was earlier unsuccessfully challenged by his lawyers.
About 270 suspects remain in detention in Guantanamo Bay.
The US considers the prisoners to be "illegal combatants", not entitled to the legal protection given to soldiers and civilians.
America's attorney general has appealed to Congress to help find a way of allowing Guantanamo detainees a civilian court appeal against their detention.
Mr Hamdan, who was captured in Afghanistan in 2001, is the first prisoner to be tried by the US for war crimes since World War II.
Despite the seriousness of the charges he faces, his three-week trial may be little more than a curtain-raiser for the military tribunal system at Guantanamo, the BBC's Jack Izzard reports from Washington.
Among the dozens of other inmates due to be tried there in the coming months are men accused of plotting the 9/11 attacks.
Human rights campaigners have accused the court of operating in a legal black hole and they and the other accused will be watching the proceedings closely, our correspondent notes.
Jury of officers
Mr Hamdan appeared in court in a khaki prison jumpsuit.
The trial judge, Navy Captain Keith Allred, began proceedings by ruling that some of the evidence obtained by interrogators while Mr Hamdan was still in Afghanistan would not be allowed during the trial.
"This military commission is assembled," he said after the jury pool was sworn in.
"You must make your determination whether or not he is guilty based solely on the evidence presented here in court and the instructions I will give you."
The trial jury is being selected from a pool of 13 US military officers and must comprise at least five members.
The verdict will require a two-thirds majority.
Mr Hamdan was captured at a roadblock in Afghanistan during the US-led invasion in 2001, two months after the 9/11 attacks on America.
Prosecutors say he belonged to Osama Bin Laden's inner circle, and was heading for a battle zone when he was arrested, allegedly with two surface-to-air missiles inside his car.
Mr Hamdan has acknowledged working for Bin Laden in Afghanistan from 1997 to 2001 for $200 (£99) a month, but denies being part of al-Qaeda or taking part in any attacks.
Mr Hamdan's defence lawyers have argued that the statements were tainted by what have been called "coercive techniques", and he was not advised of his right against self-incrimination.
His lawyers have tried to halt the trial on grounds of legality.
In June, the US Supreme Court ruled that detainees had to be able to challenge their detention in civilian courts.
But a judge ruled last week that the military tribunal could begin as scheduled on Monday without contradicting the Supreme Court.
In a speech on Monday, US Attorney General Michael Mukasey said the Supreme Court had "left many significant questions open" in its ruling.
Although the court did rule that detainees had the right to appeal in civilian courts against their detention without trial by the military, Mr Mukasey said, it "stopped well short of detailing how the habeas corpus proceedings must be conducted".