The attack occurred as the defendant coolly confessed to his crime.
No new date has been set for the trial, which is taking place behind closed doors, to resume.
Two men are being tried for the murders of 17 children and three adults in the deserts on the outskirts of Tehran over the course of a year.
'No remorse'
Reports from inside the courtroom depict a scene of chaos after the defendant, Mohammad Bijeh, started recounting how he had killed his seventh victim.
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They wanted to kill the accused men with bare hands
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Witness
"He was completely calm and free of any remorse," a witness told the news agency AFP.
"He gave all the gory details... The family of the victim then rose from their seats and ran towards him. Then other relatives of the victims began shouting and running at the accused. They wanted to kill the accused men with bare hands.
"The police then quickly whisked the accused out of the court. The courtroom was a mess and the hearing was halted," the witness said.
Father's fears
The BBC's Frances Harrison reports Iranian media as saying the killers buried some of their victims in shallow graves placing dead animals on top to disguise the smell of the decaying corpses.
The father of one of the children has now told a newspaper he thinks the killings are linked to the trade in human organs because body parts were removed from some corpses before they were burned.
The coroner told the court four victims were poisoned with cyanide, seven were strangled and six were beaten to death.
If found guilty, the men could face execution.
But some of the families of the victims, who are very poor, have asked for blood money from the killers instead of the death penalty as is their right under Islamic law, our correspondent reports.
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