Former aides of Saddam Hussein, among them the notorious "Chemical Ali", have been accused in court of massacring 100,000 people in a 1991 Shia uprising.
One witness told the Baghdad hearing how victims were led to a sports centre and killed in batches - 25 at a time.
The 15 defendants - including Ali Hassan al-Majid, or Chemical Ali - are accused of crimes against humanity.
Majid has already been sentenced to death for his role in the genocide against Kurds in northern Iraq in 1988.
On Monday, an unnamed Shia witness told the tribunal how his son had seen the 1991 slaughter take place.
Testifying from behind a curtain to protect his identity, he said Majid "was present for the execution of the first batch, and then he told his guards to continue executing the others and he left".
Majid responded: "You were not there. You only heard, you did not see. How can you be so sure? I wasn't in Basra at all at that time."
Majid also made an unsuccessful attempt to have the proceedings halted, claiming defence lawyers were too scared to attend the trial.
Poison gas
In an earlier session, witness Laila Kathum accused Saddam Hussein's troops of arresting her relatives and said Majid himself had killed her two sons by throwing them out of a helicopter.
Many Shia rebelled against Saddam Hussein, expecting support from the US Army, which had just defeated the Iraqi forces in the first Gulf War.
But US forces withdrew from Iraq and Saddam's security forces put down the rebellion.
On 4 September, Majid's death sentence in another genocide trial was confirmed by an appeals court and under Iraqi law he must be hanged within 30 days.
Majid and two other top officials in Saddam's regime had been found guilty of organising what was known as the Anfal campaign against the Kurds.
He acquired the nickname Chemical Ali during the operation after poison gas was used.
A notorious massacre at Halabja in March 1988, in which 5,000 people were killed by gas, was not included in the charges as it was seen as being separate from the Anfal campaign.
Saddam Hussein was executed in December 2006 after being convicted of ordering the execution of 148 Shia in the village of Dujail in 1982.
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