The US Supreme Court has begun hearing a case on the legality of the military trials at the Guantanamo Bay prison camp in Cuba.
Mr Hamdan says he was just a driver and not an al-Qaeda plotter
Lawyers for detainee Salim Ahmed Hamdan - Osama Bin Laden's former driver - say the tribunals are unconstitutional.
Legal experts say a potential landmark ruling against the trials would curb President George Bush's expanded powers in punishing suspected terrorists.
Mr Hamdan, from Yemen, denies charges of conspiracy to commit war crimes.
Mr Hamdan's lawyer told US Supreme Court justices in Washington that the tribunals - formally called "military commissions" - were flawed.
"This is a military commission that is literally unburdened by the laws, constitution and treaties of the United States," Neal Katyal said, according to the Associated Press news agency.
Mr Hamdan's lawyers earlier said the commissions - the first such trials since World War II - breached the 1949 Geneva Convention governing the treatment of "enemy combatants" in time of war.
The Bush administration says the conventions do not apply to al-Qaeda members.
Mr Hamdan, who has been held at Guantanamo since 2002, says he was paid a pittance as Bin Laden's driver in Afghanistan and denies he was a member of al-Qaeda.
Of the 490 suspected al-Qaeda and Taleban prisoners detained at the base, 10 people, including Mr Hamdan, face military trials.
The military commissions have been set up by the Pentagon on President Bush's orders.
Challenge to Bush
Mr Hamdan's trial was halted last year when a federal court ruled that he could only be tried by a military commission if it was proved that he was not a prisoner of war.
Before joining the Supreme Court, Chief Justice John Roberts was part of the US appeals court panel that ruled against Mr Hamdan.
Because of the conflict Mr Roberts has removed himself from the case, which will now be heard by the remaining eight members of the US' highest court.
The BBC's Justin Webb in Washington says this is seen as one of the most important cases to come before the Supreme Court since the 11 September 2001 attacks.
Legal scholars see it as a challenge to the Bush administration's assertion that the president has wide powers to fight terrorism as he sees fit - powers granted by the Constitution and by Congress.
Efforts by the White House to get the Supreme Court to drop the case have been rebuffed by the justices, who believe this is an issue they should at least consider, our correspondent says.
The court is expected to issue a ruling in early summer.