The men are charged with murder and other crimes and face a likely death sentence if convicted.
The defendants were protesting a ruling that three of them would not be allowed to speak at the session, including alleged plot mastermind Khalid Sheikh Mohammed.
US Army Colonel Steve Henley, the trial judge, previously ruled that he would only allow Ramzi Bin al Shibh and Mustafa Ahmed al-Hawsawi — the two defendants whose mental competency was in question — to speak at the hearing.
The Thursday proceeding was convened to focus on preparations for a hearing on whether those two are mentally competent to stand trial.
It has not been resolved whether a defendant can be forced to attend all sessions of the Guantanamo war crimes court. Military judges have at times directed guards to forcibly bring detainees to court, while detainees at other times have been allowed to boycott sessions.
