BAGHDAD - The postwar Iraqi tribunal trying former Saddam Hussein aides opened its third proceeding Tuesday, putting the ex-defense minister known as "Chemical Ali" and 14 other men on trial for the regime's brutal crushing of a 1991 rebellion by Shiite Muslims.

In some of the day's most dramatic testimony, a witness recalled the random shooting deaths of a teenage girl and three other people in a square packed with detainees as Saddam's troops rounded up Shiites during the uprising that followed Iraq's defeat in the Gulf War.

That account capped the opening session against the 15 defendants facing charges of crimes against humanity stemming from the 1991 crackdown's killing of tens of thousands of Shiites. Three of the defendants already have been sentenced to death in another case.

One of the three -- Ali Hassan al-Majid, Saddam's cousin and the former defense minister who gained the nickname "Chemical Ali" after poison gas attacks on Kurdish towns in the 1980s -- entered the courtroom wearing a traditional white robe and red-and-white checkered headdress.

The chief judge, Mohammed Oreibi al-Khalifa, told the men they were charged with crimes against humanity, which court officials said included murder, torture, persecution and random detentions. The crimes carry the maximum penalty of death by hanging, the judge said.

The charges stem from the aftermath of the 1991 Gulf War, in which a U.S.-led coalition drove Saddam's army from Kuwait.

Iraqi Shiites in the south and Kurds in the north -- repressed under Saddam's Sunni-dominated regime -- sought to take advantage of the defeat, launching separate uprisings and briefly seizing control of 14 of the country's 18 provinces.

U.S. troops created a safe haven for Kurds in three northern provinces, preventing Saddam from attacking. But Iraqi troops crushed the other uprising in the predominantly Shiite south.

Iyad Abdel-Zahra Ashour, a 41-year-old teacher, testified he was arrested by the military while visiting his brother in the hospital. The brother had been wounded while trying to prevent people from looting a wheat mill in the southern city of Basra, he said.

Ashour said he was held with more than 300 other detainees on a square surrounded by soldiers. He said al-Majid and co-defendant Abdul-Ghani Abdul-Ghafour, a senior Baath party official, arrived at the square and fatally shot three people, including a 14-year-old girl.

The other detainees were later transferred to prisons where they were beaten and tortured, Ashour said. "They tried to get me to confess, but I resisted because I was innocent," he said.

He said he was later released but has never learned the fate of his hospitalized brother. "The hospital officials told us that al-Majid at that time had ordered that all wounded people be executed because they were from the opposition," Ashour said.

The defendants who spoke Tuesday maintained their innocence and questioned the U.S.-backed court's credibility.

Sabir al-Douri, former director of military intelligence, told the judge he was in Baghdad during the 1991 uprising and did not visit the south during that period.

Sabawi Ibrahim, a Saddam half brother who headed the Mukhabarat intelligence agency in 1991, challenged the trial, saying the court "was established by the occupiers who ignored the international law and invaded Iraq without the permission of the United Nations."

He also defended the crackdown on the Shiite uprising, saying it was orchestrated by Iran, with which Saddam's regime had fought a devastating war.

"Iran failed to achieve its goal in the 1980-1988 war, but it seized the chance in 1991 to kill Iraqis and loot Iraq," Ibrahim said. "Iran used its elements and agents to destroy Iraq."

The day's testimony was begun by 65-year-old retired army officer Reibit Jabar Risan. He said his cousin and nephew died when the army shelled his village near Basra, Iraq's second-largest city, after villagers burned the police station at the start of the uprising.

It was the third trial of former regime officials. The first led to the hanging of Saddam and three others after their conviction for the 1982 killings of 148 Shiites from the town of Dujail.

The second trial involved the killing of more than 100,000 people during a 1980s military crackdown on Kurds.

Al-Majid and two co-defendants -- Sultan Hashim Ahmad al-Tai, a former defense minister, and Hussein Rashid Mohammed, an ex-deputy director of military operations -- were sentenced to death in that case. They are standing trial in the Shiite uprising case pending their appeals.