Eight people have been confirmed dead at Tehran's Evin prison, Iran's judiciary said on Monday.
Four more people died overnight in hospital following the fire at the jail, which is notorious for human rights abuses and poor treatment of political prisoners.
Six others are still in hospital, said news outlets affiliated to Iran's judiciary and the Islamic Revolutionary Guard Corps, which operates high-security wings in Evin where prisoners are kept in solitary confinement.
All eight were jailed for theft, the judiciary's Mizan news reported.
The fire was the result of a fight between prisoners in a sewing workshop, Tehran governor Mohsen Mansouri said.
Activists abroad and former inmates are sceptical of the official version of events.
In social media footage, gunshots could be heard as the fire blazed at the prison in the north of the Iranian capital.
On Monday, Iran's chief justice said “enemy agents” were behind the blaze and called for leading “rioters” ― a term authorities have used to refer to protesters ― to be put on public trial “as soon as possible.”
The fire damaged one of the largest buildings in the complex, according to satellite photos analysed by AP.
Satellite images show the roof burning away from a large building that is part of the prison's northern section.
The Iran Prison Atlas, a project by the California-based rights group United for Iran had previously identified the structure as housing prisoners convicted on fraud and theft cases — not those held on political charges.
However, the Iran Prison Atlas said the type of inmates housed there has changed over time.
Lawyers for political prisoners held at Evin have said their clients were also affected.
The incident occurred as Tehran tries to exert its authority amid widespread protests that have continued for the fifth week.
The protests were sparked by the death of Mahsa Amini, 22, in morality police custody on September 16. She had been detained three days earlier for wearing her hijab “improperly”.
Hundreds of people arrested during the protests, including students, have been sent to the Evin prison, which is known for holding some of the country's most prominent lawyers and activists.
Inmates have been holding sit-ins and protests in solidarity with the protest movement that has seen more than 230 killed, human rights groups have said.
Dozens of foreign citizens have been held at the prison, including Iranian-British dual citizen Nazanin Zaghari-Ratcliffe. She spent months in solitary confinement at Evin and has spoken about the abuse she suffered from prison staff.
Ms Zaghari-Ratcliffe was under house arrest when released in March.
Families of inmates gathered on Sunday near the prison hoping for news of their loved ones inside.
Masoumeh, 49, told AP that her son, 19, was taken to the prison two weeks ago after taking part in the street protests.
“I cannot trust news about his health, I need to see him closely,” she said.
Last year, leaked video footage from the prison showed guards beating inmates, prompting renewed concerns from rights groups.
The surveillance footage was likely just the tip of the iceberg, Human Rights Watch said.
Political prisoners held at Evin often face serious abuse, including prolonged solitary confinement, use of blindfolds, and torture, it said.
