Police have fired tear gas at Iranian protestors gathered in central Tehran marking Students Day.
The clashes came as a group of Iranian students issued a call for mass protests against President Mahmoud Ahmadinejad to coincide with the annual event.
A witness said the protestors were chanting 'Death to the Dictator' and 'Do not be scared. We are all together'.
Foreign media have been banned from covering today's event.
A reformist website claimed that Iran has shut down the mobile phone network in central Tehran to block supporters of Mir Hossein Mousavi communicating with each other.
'The network in central Tehran and near Tehran university is completely down,' said the Rah-e Sabz website. This has not been independently confirmed.
Students of Tehran’s prestigious Amir Kabir University had earlier issued a statement urging protests against Mr Ahmadinejad.
‘We are asking all people to come to universities so we can have one voice to protest at the coup d'etat,’ said the statement, issued by the group going under the name ‘Green university students of Iranian universities’.
Green was the signature colour of main opposition leader Mir Hossein Mousavi’s election campaign for the 12 June presidential poll.
He lost to Mr Ahmadinejad in what he claims was a ‘fraudulent’ election staged to return the hardliner to power.
Since then his supporters have taken to streets in Tehran at the slightest opportunity to demonstrate against Mr Ahmadinejad, accusing him of ‘stealing their votes’.
Hundreds of thousands of protestors poured onto streets in the immediate aftermath of the poll and in the deadly unrest that followed dozens were killed and thousands arrested.
The defiant protests shook the pillars of the Iranian regime in what was one of its worst crises since the 1979 Islamic revolution.
The elite Revolutionary Guards have warned they will crack down on any attempt by regime opponents to hijack the annual Students Day, which marks the 1953 killing by the shah’s security forces of three students, just months after a US-backed coup toppled popular prime minister Mohammad Mossadeq.
