Amateur video posted on YouTube appears to show riot police armed with batons beating up a man on a street in Tehran.

The posting says the footage was shot on Sunday.

The video also showed the skyline of Tehran with gunfire heard amid crowd noises.

A gunman is seen firing his weapon from the top of a building, while small fires burn on the street and inside the building.

Largely the streets of Tehran have sunk into sullen calm since riot police crushed huge demonstrations in which at least 20 people were killed.

On Sunday, one witness said riot police armed with batons and Basij militia scuffled with some of the more than 1,000 pro-reform Iranians gathered outside Tehran's Qoba mosque to mark the anniversary of a 1981 bombing that killed dozens of senior officials.

The report could not be independently confirmed, but an ally of election runner-up Mirhossein Mousavi played down the incident, saying only a small group had gathered amid a strong police presence.

Video posted on the Internet supposedly shot outside the Qoba mosque showed an aide of Mousavi holding up his mobile phone to a loudspeaker to enable crowds to hear Mousavi, President Mahmoud Ahmadinejad's main challenger in the disputed election.