The self-confessed mastermind of the 9/11 attacks and four co-defendants defied a military court here Saturday, with one of the accused shouting "you are going to kill us."

The five were brought into the courtroom to be arraigned for the killing of 2,976 people on September 11, 2001 when Al-Qaeda militants flew hijacked airliners into the World Trade Center in New York, the Pentagon and a field in Pennsylvania.

But the proceedings quickly turned chaotic with the men refusing to respond to the judge's questions. Defendant Ramzi Binalshibh interrupted the proceedings by suddenly standing to pray, and then alternately kneeling and standing.

He then broke his silence, shouting at the judge, Colonel James Pohl, "You are going to kill us and say that we are committing suicide."

Khalid Sheikh Mohammed's lawyer David Nevin had said his client, who three years go confessed to the 9/11 attacks "from A to Z," probably would not speak at the hearing because he is "deeply concerned by the fairness of the process."

Dressed in white jumpsuits and some wearing white turbans, the men were making their first public appearance in more than three years. Mohammed sported a flowing brown beard.

Only one, Walid bin Attash, was handcuffed when the group was brought into court, but Pohl ordered the manacles removed after being assured he would "behave appropriately."

"Prosecution is ready to proceed in the case of Khalid Sheikh Mohammed," chief prosecutor Brigadier General Mark Martins told the military tribunal.

The arraignment, one of the last steps before a so-called "trial of the century" takes place, marks the second time the United States has tried to prosecute the 9/11 suspects.

It comes more than a decade after the most lethal attacks on US soil in modern history, and about one year after President Barack Obama ordered the US Navy SEALs raid that killed the man behind it all -- Osama bin Laden.

Also in court at Guantanamo were Mohammed's Pakistani nephew Ali Abd al-Aziz Ali -- also known as Ammar al-Baluchi -- and Mustapha al-Hawsawi of Saudi Arabia.

The five men have been held for years at the US naval base in southern Cuba while a legal and political battle has played out over how and where to prosecute them. Debates have also raged over their treatment.

Mohammed was arrested in 2003 and spent three years in secret CIA jails where he was subjected to harsh interrogations, including waterboarding, and confessed to a series of attacks and plots.

In the specially designed courtroom the men studiously ignored the proceedings for much of the morning, gazing down at their feet or appearing to read the Koran.

"Accused refused to answer," Pohl repeated over and over again, each time an accused refused to respond to questions.

Meanwhile, the proceeding bogged down over issues such as the translation of exchanges, the pronunciation of the names of the accused and even the rank of military lawyers.

In a sign of the acute public interest in the proceedings, the Pentagon has opened four military bases in the United States to allow families of the 9/11 victims to watch the case unfold on a giant screen.

Out of 200 applicants, 60 journalists obtained seats for the hearing, while another 30 are covering the event from Fort Meade in Maryland using a closed-circuit television feed.

Obama, a Democrat, wanted to hold the trial in a civilian court in Manhattan, just steps from Ground Zero, where the World Trade Center's Twin Towers once stood. But stiff Republican opposition in Congress scuttled those plans.

The military tribunal system, however, has come under scrutiny, and some of the sharpest criticism has come from former chief prosecutor Morris Davis.

"History will judge this as a mistake," he said.

The time that has lapsed since the attacks and the arrest of their presumed authors is a concern likely to come up at the hearing, said Ali's attorney James Connell.

The trial could still be years away, unless Mohammed pleads guilty to be put to death sooner and become a "martyr" for Al-Qaeda.

Cliff Russell is among the small number of victims' relatives who won a lottery to attend the arraignment proceedings. His firefighter brother was killed when the World Trade Center's twin towers collapsed in New York.

"I'm not looking forward to taking somebody's life... but it's the most disgusting, hateful awful thing I ever could think of; it's crazy," said Russell.