A Swedish court held trial of a 60-year-old Iranian suspected of war crimes and murder, Swedish news agency TT reported.
About 100 demonstrators gathered outside the court in Stockholm on Tuesday to protest against the Islamic Republic of Iran.
Hamid Noury has been in custody in Sweden for almost two years and is accused of having played a leading role in the killing of political prisoners executed on government orders at the Gohardasht prison in Karaj, Iran, in 1988.
He denies the accusations, according to prosecutors who announced the charges last month.
It is the first time anyone has been brought before a court to stand trial over the mass murders.
Noury and others "organised and participated in executions by selecting which prisoners should appear before a court-like commission, which had the job of deciding which prisoners should be executed", prosecutor Kristina Lindhoff Carleson told the court, according to TT.
She also read out the names of 110 people whose executions Noury is accused of helping to orchestrate. Under Swedish law, courts can try Swedish citizens and other nationals for crimes against international law committed abroad.
Iran's President Ebrahim Raisi, who was inaugurated last week, had his involvement in the case as one of four judges who oversaw the 1988 killings, activists have been warning.
When asked about the allegations, Raisi told reporters after his election in June that he had defended national security and human rights.
"If a judge, a prosecutor has defended the security of the people, he should be praised ... I am proud to have defended human rights in every position I have held so far," he said.