Iranian police fired teargas during clashes with opposition supporters trying to stage a demonstration in central Tehran on Wednesday and made several arrests, witnesses said.

The clashes took place in central Tehran's Haft-e-Tir square where groups of opposition supporters had gathered for a protest, even as thousands of Iranians were staging an anti-American rally outside the closed US embassy to mark the 30th anniversary of the storming of the compound by Islamist students.

Witnesses said police beat the opposition supporters in a bid to break up the rally but the crowd of several hundred refused to move.

Opposition supporters have since June been staging protests in Tehran against the re-election of President Mahmoud Ahmadinejad in a presidential they claim was massively rigged.

Protesters on Wednesday chanted "Death to the dictator" while a pro-government group which had also gathered at the square chanted "Death to America."

The June 12 re-election of Ahmadinejad triggered the worst political crisis in the 30-year history of the Islamic republic.