Online Iranian videos appeared to show smoke rising on Saturday from Tehran's Evin prison, which holds political prisoners, as shots and an alarm could be heard.
There was no immediate comment from Iranian officials or reports from state media on the footage, which was shared on social media.
"Gunshots can be heard from Evin Prison and smoke can be seen," said the activist website 1500tasvir, which also shared video footage it said showed special forces on motorbikes heading for the prison.
"Families of prisoners have gathered in front of the main door of Evin prison," said a witness contacted by Reuters. "I can see fire and smoke. Lots of special forces. Ambulances are here too."
Siamak Namazi, an Iranian American imprisoned in Iran for nearly seven years on espionage-related charges rejected by Washington as baseless, returned to Evin on Wednesday after being granted a brief furlough, his lawyer said. Other dual nationals are also held at Evin.
The prison, which mostly holds detainees facing security charges, has long been criticised by Western rights groups and it was blacklisted by the U.S. government in 2018 for "serious human rights abuses".
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.
(Reporting by Dubai newsroom, Editing by William Maclean)
