Video appears to show the sky lit up orange as flames are seen behind a tower, and smoke billows.
Image source, TwitterA fire has broken out at Iran’s notorious Evin prison, with footage posted online showing flames and smoke billowing from the area.
Gun shots and alarms have been reported as coming from the jail, the primary site for detaining political prisoners.
State media say calm has been restored to the prison, and has blamed “criminal elements” for the blaze.
The fire comes as Iran continues to be rocked by its most intense unrest in decades.
Protests have been taking place across the country once again on Saturday.
They first erupted last month when anger over the death in police custody of 22-year-old Kurdish Iranian Mahsa Amini boiled over. Officials say she died from an underlying health condition, but her family say she died after being beaten by morality police.
It is unclear what was the cause of the fire at Evin prison, but there are further reports that special forces have been deployed to the area. Firefighters are at the scene, according to state media.
A witness told Reuters news agency that “families of prisoners have gathered in front of the main door”.
They added: “I can see fire and smoke. Lots of special forces. Ambulances are here too.”
As well as political prisoners, journalists, and many dual and foreign nationals are also imprisoned in Evin.
British-Iranian dual nationals Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori were both held on spying charges, which they denied, before their release earlier this year. They were imprisoned for six and five years respectively.
The prison has long been criticised by Western rights groups. Human Rights Watch has accused authorities at the prison of using threats of torture and of indefinite imprisonment, as well as lengthy interrogations and denial of medical care for detainees.