Fifteen aides of Saddam Hussein have been accused in a court in Iraq of "one of the ugliest crimes ever committed against humanity in modern history".
"Chemical Ali" has already been sentenced to death
The defendants are alleged to have helped suppress a Shia uprising after the 1991 Gulf War, in which tens of thousands are thought to have died.
In recent years, mass graves containing hundreds of bodies have been uncovered.
Those in the dock include the cousin of Saddam Hussein, Ali Hassan al-Majid, who is widely known as "Chemical Ali".
Majid has already been sentenced to death following an earlier trial for genocide against Iraq's Kurdish population in the so-called Anfal campaign of 1988.
Two more of the defendants in the latest trial - Sultan Hashim al-Tai, a former defence minister, and Hussein Rashid al-Tikriti, a former deputy chief of operations for the armed forces - were also sentenced to death for those killings.
Dressed in a cream robe and a white kuffiya shawl, Majid was among the first to enter the Iraqi High Tribunal in Baghdad's heavily-fortified Green Zone.
"I am the fighter Ali Hassan al-Majid," he replied when asked to identify himself by Judge Mohammed al-Oraibi al-Khalifa.
Once they had been seated, the 15 men were told they faced charges of crimes against humanity "for engaging in widespread or systematic attacks against a civilian population", offences punishable by death.
Ali Hassan al-Majid
Sultan Hashim al-Tai
Hussein Rashid al-Tikriti
Abd Hamid Mahmoud al-Nasseri
Ibrahim Abdul Sattar al-Dahan
Walid Hamid Tawfik al-Nasseri
Iyad Fatiya al-Rawi
Sabawi Ibrahim al-Hassan
Abdul Ghafour Fulayih al-Ani
Ayad Taha Shihab al-Douri
Latif Maal Hamoud al-Sabawi
Qais Abdul Razzaq al-Adhami
Sabir Abdul Aziz al-Douri
Saadi Tuma Abbas al-Jabouri
Sufyan Maher al-Ghairiri
"The acts committed against the Iraqi people in 1991 by the security forces and by the defendants sitting were among one of the ugliest crimes ever committed against humanity in modern history," the chief prosecutor said in his opening remarks.
The prosecutor alleged that former President Saddam Hussein had ordered tanks and infantry troops withdrawn from Kuwait to attack Iraq's southern provinces when he realised he had lost control there.
"The convict Saddam made rash judgements. Majid was authorised to demolish anything and kill anyone who came in the way of the forces," he said.
He then accused the defendants of both ordering and carrying out cold-blooded executions while they directed Baghdad's military response to the uprising.
"The helicopters were bombing the cities and houses of people. Prisoners captured were killed," he said.
"Majid used to come to detention centres, tie the hands of the detainees and then shoot them dead with his weapon. The dead were then later buried in mass graves.
"Many mass graves have been found since the 2003 war ended. And we will find many more if we keep searching."
The first witness, 65-year-old former soldier Raybath Jabbar Risan, said troops from the elite Republic Guard had shelled his village in Basra province with artillery and mortars.
"My cousin was killed and nephew wounded. My brother's house was burned. I escaped with my family," he told the court.
"I worked in the army for 30 years and never imagined they would do this to me and my family."
The court expects to hear about 90 witnesses as well as audio tapes and written reports. US officials said there was little remaining evidence of the orders given, however, as Saddam Hussein ordered the destruction of records.
The Shaaban Intifada (Uprising) started in March 1991 as defeated Iraqi troops fled back to southern Iraq after US-led forces took control of Kuwait.
Galvanised by a message by US President George Bush to "take matters into their own hands", the Shia strongholds of Najaf and Karbala rose in revolt in an attempt to topple Saddam Hussein.
Soon, thousands of rebel troops seized control of the city of Basra and 14 of Iraq's provinces, and advanced to within 60 miles of Baghdad.
But despite these early successes, the rebellion was swiftly crushed by government forces. Mass reprisals followed in which tens of thousands of people are believed to have died.
Many Shia blame President Bush for the uprising's failure, as the US came to a ceasefire agreement that allowed forces loyal to Saddam to crush the rebellion by using helicopter gunships.