Rizgar Mohammed Amin, the Kurdish judge trying Saddam, entered the court shortly before 11.30 am (0830 GMT) on Wednesday.
Saddam, who had earlier told judges to “go to hell”, followed 10 minutes later with his seven co-defendants.
Amin said he planned to call five witnesses in what was expected to be the final hearing of the year before proceedings are adjourned for about a month.
Prosecutors accuse Saddam and the others of ordering the killings of 148 people from the town of Dujail, north of Baghdad, following a failed attempt to assassinate him there in 1982.
Liable to be hanged if found guilty, Saddam is the first Middle East head of state to be tried by his own people.
So far, the trial has heard from 10 prosecution witnesses who have told the court of the torture, beatings and hardships they suffered under Saddam in the wake of the Dujail killings.
The court is set to hear five more
Eight have testified from behind a curtain out of fear for their lives, and their names were withheld from the court.
But the first witness called on Wednesday appeared openly, standing just yards (metres) from Saddam who took notes and followed proceedings from inside the caged defendants’ dock at the heavily fortified Baghdad courtroom.
The witness gave his name as Ali Hasan al-Haidari and said he was 14 at the time of the Dujail massacre.
Dressed in a brown suit and white shirt, he spoke calmly and coherently, telling the court his brother was executed under Saddam and his family had been rounded up after the killings.
He said he was taken to the headquarters of Saddam’s Baath Party in Dujail where he saw nine corpses lying outside.
“I recognised all of them,” he said, before listing names of the alleged victims.
He said he was then taken to the headquarters of Saddam’s intelligence service in Baghdad where he saw horrific torture.
“A man would leave on his feet and come back thrown in a blanket”
Guards applied electric shocks to detainees, and heated up plastic tubing and allowed the hot plastic to drip on to the bodies of their victims, he said.
“They would be in such pain as the plastic solidified on their bodies,” he recalled. “A man would enter on his feet and get out carried in a blanket.”
His testimony was among the strongest heard so far in the stop-start and often chaotic trial, which opened on 19 October but has been adjourned three times.
Al-Haidari also made a direct accusation against Saddam’s co-defendant, half-brother and feared former intelligence chief, Barzan Ibrahim al-Tikriti.
He said Barzan had been present in the building where the torture had taken place and on one occasion had kicked al-Haidari hard as he lay suffering from a fever.
“I was in pain for weeks because of that kick,” he said.