The arraignment will launch the highest-profile test yet of a tribunal system that faces an uncertain future. The US Supreme Court struck down an earlier system as unconstitutional in 2006, and is to rule this month on the rights of Guantanamo prisoners, potentially delaying or halting the proceedings.

And with less than eight months remaining in US President George W. Bush's term, presidential candidates Barack Obama and John McCain both say they want to close the military's offshore detention center.

Dozens of US and international journalists arrived at Guantanamo on Wednesday on a military plane from Andrews Air Force Base outside Washington, joining prosecutors, defense attorneys and observers who arrived earlier at the Navy base.

Mohammed and the four alleged coconspirators all face possible death sentences. They are expected to be seated Thursday morning at separate defense tables aligned in a row inside the prefab courthouse. Many of the participants and observers will stay nearby in tents erected on an abandoned airport runway as part of the "expeditionary" legal complex.

Family members of those killed on September 11, 2001, wanted to attend, but the military said it was too difficult logistically to accommodate dozens more people. Instead, the military is planning to show the trial but not the arraignment on closed-circuit television to victims' families gathered on US military bases.

"For transparency and to add legitimacy to the trial, they should have the loved ones there," said Dominic J. Puopolo, whose mother Sonia Morales Puopolo was a passenger on American Airlines Flight 11, the first jet that crashed into the World Trade Center.

Puopolo said he also wanted to see the defendants, especially Mohammed, who claimed he personally proposed the plot to Osama bin Laden.

"This is an architect of such pure evil," Puopolo said, "I want to see him eye to eye."

Even without televised coverage of his arraignment, Mohammed's first public appearance since his capture in 2003, the US is taking a security risk by giving him an opportunity to spread al-Qaida propaganda, said Benedetta Berti, a research fellow at Tufts University's Jebsen Center for Counter-Terrorism Studies.