Prosecutors are set to outline the case against a former Khmer Rouge leader in Cambodia for the first time, 30 years after the brutal Maoist regime fell, BBC reported.
The UN-backed trial of Kaing Guek Eav, known as Comrade Duch, opened in Phnom Penh last month but is now set to hear from prosecution lawyers and witnesses.
Duch ran a notorious prison camp and is accused of presiding over the murder and torture of at least 15,000 inmates.
The Khmer Rouge killed up to two million people in less than four years.
Duch is the first Khmer Rouge leader to face the tribunal - with four more of the regime's senior figures in custody and awaiting trial.
He is the only one who has admitted his part in the atrocities and asked for forgiveness from his victims.
Analysts say his testimony could be vital for the trials of the four other leaders.
The BBC's Guy DeLauney, in Phnom Penh, says Duch will give his side of the story in the coming weeks.
But before that happens, our correspondent says, he will have to hear the charges against him - which could take a full day.
The hearings come after a decade of painstaking negotiations between politicians, judges, lawyers and Cambodians.
In a statement, human rights group Amnesty International welcomed Duch's trial.
"The Cambodian people will finally see one of the most notorious Khmer Rouge leaders face trial," the statement said.
"But many more need to face the court to really deliver justice to the millions of victims of these horrific crimes."
The Khmer Rouge took over Cambodia in 1975, ousting a US-backed government shortly after the US pulled out of neighbouring Vietnam.
Driven on by Maoist principles, they attempted to create a peasant society by systematically emptying the cities and forcing the population to work in the fields.
By the time the Vietnamese army invaded and overthrew the Khmer Rouge in 1979, the regime had executed, starved and overworked up to two million Cambodians to death.