Reports from Iran say the security forces have fired live ammunition in the vicinity of anti-government demonstrators in the capital, Teheran.
There are no reports of whether anyone was wounded.
State media confirmed there had been clashes. However, a ban on foreign media means details are hard to verify.
The BBC reports the violence came on the day that Iran holds an annual commemoration for the killing of three students in 1953.
Earlier, riot police clashed with protestors around Tehran university.
Iranian security forces, including the elite Revolutionary Guards, previously warned that they would step in to prevent any attempt to use the event to stage opposition protests.
The Rah-e Sabz website says authorities have shut down the mobile phone network in central Tehran to stop opposition protesters from contacting each other.