Iranian police have clashed with opposition supporters in central Tehran, according to witnesses.
Police used batons and tear gas, according to the witnesses. The reports could not immediately be verified because of a foreign media ban, the BBC reports.
The reported clashes came as Iranians prepared to hold an annual rally to mark the killing of students in 1953.
Earlier, hundreds of Iranian police were reported to have surrounded Tehran University to try to block protests against President Mahmoud Ahmadinejad who won a disputed election in June.
Opposition groups reject Mr Ahmadinejad's re-election in the poll on 12 June, saying it was rigged to ensure his return.
Police and elite Revolutionary Guards have warned that any "illegal" rally will be fiercely confronted.
Iran has banned foreign media from reporting on the Student Day rally. Journalists are not allowed to leave their offices to cover stories between Monday and Wednesday.
Iranian authorities have also reportedly cut internet access in central Tehran.