Baton-wielding Iranian police have clashed with mourners holding memorials for those killed in post-election violence, reports say.
State TV said police used teargas to disperse crowds from the grave of Neda Agha Soltan, whose death became a symbol of post-election unrest.
Opposition leader Mir Hossein Mousavi tried to join the mourners but police forced him to leave, witnesses said.
Further confrontations were reported at a second gathering in central Tehran.
Several hundred people defied a heavy police presence to gather at the Grand Mossala prayer area, witnesses 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 the Grand Mossala, 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.
“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,” a witness told Reuters.
Mr Mousavi was surrounded by police shortly after he arrived, witnesses said.
“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,” one person told AFP.
Some people in the crowd threw stones and chanted anti-Ahmadinejad slogans, reports said, as security personnel with batons charged at them.
One man told the BBC there were about 3,000 people there. Seven or eight men used professional cameras to film the protesters, he said.
Shortly afterwards hundreds more demonstrators were said to have gathered at the Grand Mossala.
Police again moved in to break up the crowds, some of whom set rubbish bins on fire, witnesses said.
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.