Iranian state TV is carrying unconfirmed reports that at least 13 people were killed in clashes during protests on Saturday, BBC reported.
It said "rioters" set two petrol stations and a mosque on fire, and attacked a military post.
State media earlier said calm had returned to Tehran, and quoted police as saying they had restored order.
It was unclear whether fresh protests would go ahead on Sunday, amid official demands for them to stop.
The protests were sparked by disputed presidential elections, but have since escalated into a political crisis striking at the heart of the establishment.
Eyewitness accounts say the police used batons, tear gas and water cannon to quash Saturday's protests, and protesters had also relayed unconfirmed reports of deaths.
The BBC and other foreign media are subject to heavy restrictions which have prevented reporters from leaving their offices to confirm many reports.
Protest leader and defeated presidential candidate Mir Hossein Mousavi has been quoted as saying peaceful protests should continue and that people have a constitutional right to demonstrate.
US President Barack Obama has warned Iran to stop all "violence and unjust action against its own people".