Iranian security forces and opposition activists have clashed in the centre of the capital Tehran, according to reformist websites and witnesses.
Some reports say shots have been fired in the air to disperse demonstrators.
The opposition had been planning to use Shia religious festivals this weekend to show their continued defiance of President Ahmadinejad's government.
Tensions have risen since influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri died a week ago aged 87.
Clashes were reported after his funeral in Qom, and in other cities since then.
Police chief Esmail Ahmadi Moghaddam has been quoted as warning that police would crack down hard against any attempts to use Shia religious processions to stage fresh protests against Mahmoud Ahmadinejad's government.
Opponents insist there was massive rigging when Mr Ahmadinejad was re-elected in June - a charge he rejects.
BBC Tehran correspondent Jon Leyne says opposition sources are building a picture of increasingly widespread and violent clashes.
IT WOULD BE NICE IF OBAMA, HILLARY AND KERRY SUPPORTED THE REBELLION THIS TIME.