GUANTANAMO BAY NAVAL BASE, Cuba — A professed September 11, 2001, mastermind, Khalid Sheikh Mohammed, took center stage in a military court yesterday as he questioned the judge’s impartiality and acted as the de facto spokesman for his four co-defendants.

Mr. Mohammed, the highest-profile Al Qaeda figure in American custody, boasted at a 2007 closed hearing that he was responsible for 31 terrorist plots and the September 11 attacks “from A to Z” — claims that American officials said were exaggerated.

Mr. Mohammed’s interactions with the judge and his co-defendants yesterday underscored his taste for the limelight and sense of authority. The former Al Qaeda no. 3 has led his co-defendants in raising challenges to the court and even assisted in getting a boycotting co-defendant to leave his cell.

Glaring at Judge Ralph Kohlmann from beneath bushy eyebrows and a black turban, Mr. Mohammed pressed the Marine colonel to explain how he could provide a fair trial as a member of the U.S. Armed Forces that are at war with Al Qaeda.

“How can you, as an officer of the U.S. Marine Corps, stand over me in judgment?” Mr. Mohammed, who is acting as his own lawyer, asked in English. “How can you be unbiased, given your position?”

Mr. Mohammed also questioned the judge about his religion, his Marine training and his knowledge of waterboarding, and other harsh interrogation tactics Mr. Mohammed experienced in CIA custody before he and his co-defendants were transferred to this American military base in southeast Cuba in September 2006.