Hamid Noury is accused of helping to kill “large numbers of prisoners” in Iran on the order of Ayatollah Khomeini in 1988, according to Swedish prosecutors.
The trial of a former Iranian official, accused of being involved in mass executions and war crimes in the 1980s, began on Tuesday in Sweden.
Hamid Noury, 60, was arrested in November 2019 at Stockholm-Arlanda International Airport during a visit to Sweden.
He has been in pre-trial detention since then.
Noury is accused of helping to kill “large numbers of prisoners” on the order of Ayatollah Khomeini in 1988, according to the Swedish prosecutors office.
This occurred at the final phase of the Iran-Iraq war in the 1980s.
The trial is expected to last for several months, and is highly sensitive, as top Iranian officials, including current president Ebrahim Raisi, have been accused of taking part in the atrocities.
Amnesty International has noted previously that no Iranian official has faced charges over the mass 1988 executions.
According to the Swedish prosecutor's office, between 30 July and 16 August 1988, Mr Noury "intentionally took the lives of a very large number of prisoners who were sympathetic to or belonged to the People's Mojahedin".
The People's Mojahedin, a political-militant organisation, were involved in frequent attacks in Iran, and advocated overthrowing the government of the Islamic Republic of Iran.
They were the target of an execution order from Ayatollah Khomeini, the founder of the Islamic Republic of Iran.
The Swedish Prosecution Authority said the suspect worked in July-August 1988 as an assistant to the deputy prosecutor in the Gohardasht prison outside the Iranian city of Karaj and allegedly took part in severe atrocities there.
Ayatollah Khomeini, according to the prosecutors, issued an order for the execution of all prisoners in Iranian jails who sympathised and remained loyal to the Mujahedin organisation.
Due to that order, a large number of prisoners were executed in the Gohardasht prison between July 30 and August 16, 1988, the prosecutors said.
His trial opened on Tuesday morning in a Stockholm court. The accused, dressed in a light-coloured suit, appeared calm, smiling easily, accompanied by his two lawyers.
Three days of hearings are scheduled this week for a trial which is due to last until April 2022. Dozens of witnesses will be called to the stand.
Several hundred people - mainly supporters of the People's Mojahedin - gathered outside the court before the trial began, urging Swedish and international justice to convict the new Iranian president Ebrahim Raissi.
Human rights NGOs have been campaigning for years for justice for what they consider to be the extrajudicial execution of thousands of mostly young Iranians across Iran as the Iran-Iraq war was ending.
During the same period, Hamid Noury is also suspected of having participated in the execution of other prisoners on the basis of their ideology or belief, considered to be opposed to the "theocratic Iranian state", according to the prosecution.
Today, the accused is the target of some thirty civil party complaints - victims, witnesses or relatives of the victims.
He "denies any accusation of involvement in the alleged executions of 1988", according to his lawyer Thomas Söderqvist.