Iranian police have arrested mourners who had gathered at a cemetery in Tehran for a memorial to those killed in post-election violence, reports say.
Opposition leader Mir Hossein Mousavi tried to join mourners at the graveside of Neda Agha Soltan, whose death became a symbol of post-election unrest.
But reports say he was forced to leave the cemetery shortly after his arrival.
Other witnesses said that mourners clung to his car, chanting "Mousavi, we support you".
Police used batons to disperse several hundred people, some of whom threw stones at police, reports said.
Opposition supporters allege the 12 June election results were rigged in favour of President Mahmoud Ahmadinejad.
Anger at the outcome led to the largest mass protests seen in Iran since the 1979 revolution which brought the current Islamic regime to power.
Neda Agha Soltan, 27, was shot dead on 20 June as she watched protests against the poll result. Her death - one of 10 that day - was filmed on a mobile phone and broadcast around the world.
Shia Muslims traditionally mark 40 days after a death with a ceremony called the "arbayeen".
Mr Mousavi and another opposition leader, Mehdi Karroubi, had asked the interior ministry for permission to hold a memorial service in Tehran's Mosalla mosque, according to an aide to Mr Mousavi, but permission was denied.
So the opposition leaders said they would join Neda's family at her graveside at the Behesht-e Zahra cemetery.
Witnesses said about 150 police as well as members of the Islamic Basij militia were there, with a number surrounding the section where Neda is buried.
Mr Mousavi got out of his car and walked up to Neda's grave, but was then surrounded by police, the witnesses told AFP news agency.
"Mousavi was not allowed to recite the Koran verses said on such occasions and he was immediately surrounded by anti-riot police who led him to his car," a witness said.
"Hundreds have gathered around Neda Agha Soltan's grave to mourn her death and other victims' deaths... police arrested some of them ... dozens of riot police also arrived and are trying to disperse the crowd," another witness told Reuters.
Iranian authorities banned all opposition protests following post-election violence.
And, reports the BBC's Jon Leyne, the authorities are particularly sensitive about these "arbayeen'' turning into political demonstrations.
That is exactly what happened during the Islamic Revolution 30 years ago in a cycle that helped lead to the downfall of the Shah, our correspondent says.
On Tuesday, officials said about 140 people detained during the protests had been released from Evin prison.
But about 200 others, accused of more serious crimes, remain in jail.
Bowing to pressure about the treatment of detainees - some of whom are reported to have died in prison - officials said that more prisoners accused of minor offences would be released on Friday.
However, 20 people have been charged with more serious offences, including bombings, carrying weapons and attacking security forces.
Tehran's public prosecutor's office has announced that the first trials of "rioters" will begin on Saturday, the official Iranian news agency Irna reported.
On Wednesday, US Secretary of State Hillary Clinton said she deplored the way the Iranian government was treating those it had imprisoned after the violence and urged authorities to release political detainees.
Mr Ahmadinejad is to be officially approved as Iranian president on 3 August.