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.
