File Photo: Islamic Republic News Agency, IRNA, on Sunday, Oct. 16, 2022, a charred building is seen after a fire on the property of the Evin prison, in Tehran, Iran. AP
The unrest at the central prison in the northern city of Karaj began Saturday when prisoners protested the transfer of an inmate to solitary confinement ahead of execution, the Norway-based group Iran Human Rights (IHR) said.
Iranian authorities have offered a different account, saying conflict erupted between prisoners convicted of drug offences, with one person killed after a stone was thrown at him.
It is the third major incident in an Iranian jail since the September 16 death in custody of Mahsa Amini that sparked a nationwide protest movement.
IHR reported prisoners had chanted slogans against the regime including "death to the dictator", referring to Iran's supreme leader Ayatollah Ali Khamenei.
They also blocked entrances to the Karaj prison and broke surveillance cameras during the riot, IHR said.
It identified the slain inmate as Mohsen Mansouri, saying he had been shot in the head.
Seventeen others had been hospitalized with injuries, the group added.
The judiciary said several inmates had minor injuries.
The identity of the inmate transferred to solitary confinement and their possible execution date were not immediately clear.
Karaj, capital of Alborz province, has been a major centre for protests.
Some people arrested in the authorities' crackdown are held in the city's central prison, but so are many convicted on unrelated offences.
Many of the protesters arrested in Karaj are held in the notorious Gohardasht prison on the city outskirts, also known as Rajai Shahr.
Alborz province's judiciary chief Hossein Fazeli Harikandi said the conflict had started in the narcotics offences ward and a fire began when prisoners set blankets alight, adding that calm had since been restored.
In October, a fire ripped through part of Evin prison in Tehran and a riot erupted at a facility in the northwestern city of Rasht.
Iranian authorities have arrested at least 14,000 people in the crackdown on the protest movement, according to the United Nations, in mass detentions activists say have caused immense overcrowding in jails.
Amnesty International said on Friday that at least 26 people were at risk of execution in connection with the protests.
Iran earlier this month executed two protesters, in a move decried by the international community.