At least 2,000 opposition supporters had gathered at the memorial for victims of the recent unrest that followed June’s disputed election.
Eyewitnesses say police dispersed the crowd and forced defeated presidential candidate Mirhossein Mousavi to leave the ceremony. Supporters chanted and surrounded the opposition leader’s car.
Mousavi was joined by another defeated presidential candidate, Mehdi Karoubi. Both had defied a police ban ordering them not to attend after accepting an invitation from the mother of Neda Agha Soltan who died in clashes last month.
The 26-year-old music student’s death became the focus for the fledgling opposition movement.
Latest reports say another large crowd of opposition supporters, who claim last month’s election was rigged, have also gathered in central Tehran.
The disputed vote has plunged Iran into its biggest internal crisis since the 1979 revolution and exposed deep divisions within the ruling elite.