A former Iranian official accused of crimes against humanity is expected to give evidence under oath for the first time on Tuesday at a landmark case in Stockholm.
Hamid Nouri, 60, is on trial over his alleged involvement in a 1988 prison massacre in Iran. He denies the charges.
The killings were allegedly an act of revenge ordered by supreme leader Ayatollah Ruhollah Khomeini against members of the Mujahideen-e-Khalq, who supported the 1979 revolution but then turned against the new leadership and fought for Iraq under Saddam Hussein, during the Iran-Iraq war.
The order came just a few days after the end of the 1980-1988 war.
Human rights groups say about 5,000 people were killed in prisons across the country, but the opposition claim that the number was six times that figure.
In 2018, the UN deemed the massacre a "crime against humanity".
Charged with more than 100 murders and grave war crimes, Nouri is expected to give evidence that allegedly implicates Iran's current president Ebrahim Raisi.
Arrested in Sweden in 2019, Nouri is scheduled to start at least three days of evidence today, more than three months after the start of a trial that has heard testimonies from victims who described how Nouri played a key part in the lining up of prisoners calling out the names of those about to be hanged.
During the mass executions of the Iranian opposition group, Nouri was an assistant to the deputy prosecutor at the Gohardasht prison near Tehran.
During the trial, former inmates at Gohardasht described how Nouri celebrated some of these executions by offering sweets to prison guards.
"He held a box of pastries and offered sweets to prison guards as they passed by”, a witness told the court.
However, Nouri claims he was not working at the prison at the time of the killings.
Observers said the 60-year-old has interrupted sessions of the court several times to complain about protesters outside the court calling for justice for senior members of the Iranian government over their alleged involvement in the mass executions.
The trial even had to relocate to Albania for a few weeks to hear evidence from seven witnesses unable to travel to Sweden.
Opposition groups have said they will stage larger demonstrations outside the courtroom on November 23 when Nouri gives evidence.
The trial is expected to conclude in April of next year.