Ali Hamza Ahmad Sulayman al Bahlul, 33, of Yemen, is charged with conspiracy to commit war crimes.
"As God as my witness, and the United States did not put any pressure on me, I am an al Qaeda member," the detainee said through an Arabic interpreter, his head shaved and wearing tan pants and a gray polo shirt.
He started to speak about his relationship to the Sept. 11 attacks, but was cut off by the presiding officer, Army Col. Peter E. Brownback.
Earlier, when al Bahlul was asked if he had any questions, he replied: "Am I allowed to represent myself?"
Brownback initially said the order setting up military commissions does not allow for it. But he later appeared willing to accommodate the request, calling a recess to consider it.
The hearings are the first step to military commissions, or trials, to be heard by a five-member military panel — the first such proceedings since World War II.
Osama bin Laden's chauffeur, 34-year-old Salim Ahmed Hamdan of Yemen, declined to enter a plea in the first hearing Tuesday. David Hicks, a 29-year-old Australian cowboy accused of fighting with Afghanistan's ousted Taliban regime, pleaded innocent Wednesday.
Al Bahlul is accused of being a "key al Qaeda propagandist who produced videos glorifying the murder of Americans to recruit, inspire and motivate other al Qaeda members" to attack the United States and other countries.
Because of a government delay in approving clearance for a translator, however, defense attorney Navy Lt. Cmdr. Philip Sundel said he hasn't had time to prepare for Thursday's hearing as challenges to the fairness of the proceedings continued to mount.
"There's virtually no chance he can get a fair trial," Sundel, Bahlul's Pentagon-appointed lawyer, told CBS News Correspondent Mark Strassmann in an interview earlier this month.
Al Bahlul's father, Hamza Ahmed, told The Associated Press in previous interviews in Yemen that the family has suffered from his son's detention, both "psychologically and financially."
"He is cultured and peace-loving and he speaks English and enjoys reading and writing poetry," Ahmed said, noting his son used to send money home.
He said his son, who is married and has four children, told him in a letter that Pakistan handed him over to the Americans and that he had left Pakistan to seek medical treatment for his grandson before the Sept. 11, 2001, terror attacks.
"In his letters he told me how much he missed his wife and children. He has not committed any crimes and he hates no one," Ahmed said.