Hamid Noury, a former Iranian prison official, had been convicted and jailed in Sweden under the principle of universal jurisdiction which allows states to try crimes carried out in other countries. It remains the only such case concerning atrocities in Iran.
The freeing of Noury is "shameful and unjustifiable" and "an affront to the Swedish judiciary", said the National Council of Resistance of Iran, the political wing of the People's Mujahedin (PMOI/MEK), outlawed in Iran, whose members accounted for the vast majority of the victims of the prison massacre.
It warned that the release of Noury would only encourage the Iranian authorities "to step up terrorism, hostage-taking, and blackmail".
Activists accuse the Iranian authorities of executing thousands of dissidents, mostly MEK supporters, in its prisons in 1988 as the war against Iraq came to a close.
Among those blamed for the massacres was former president Ebrahim Raisi, who died in a helicopter crash last month, accused by activists of having served on a four-man "death committee" that approved the executions.
As well as the MEK, several rights groups had applauded Sweden's arrest of Noury on its territory and supported the campaign to bring him to justice.
Mahmood Amiry-Moghaddam, director of Norway-based NGO Iran Human Rights, said Sweden's move had rewarded "hostage takers and criminals".
"The message to the Iranian regime is that no matter what crimes you commit, we are willing to engage with you. This decision will further endanger all Western citizens travelling to Iran and neighbouring countries," he told AFP.
"The release of Noury marks a shameful chapter in the history of the Swedish government," he added.
Shadi Sadr, founder of London-based NGO Justice for Iran which seeks accountability for crimes committed in Iran, said that "regardless of the motivation" for Sweden's move "his release is a blatant, shameless act that has left countless victims and the wider Iranian society in shock."
Noting that his jailing was the "first and only" universal jurisdiction case involving crimes in Iran, she said "it raises serious questions about political influence on universal jurisdiction cases".
Iran freed EU diplomat Johan Floderus, who has been held since April 2022 and faced the death penalty on spying charges, and Saeed Azizi, who was arrested in November 2023. Noury, meanwhile, had been serving a life sentence.
