Article published on the 2009-11-04 Latest update 2009-11-04 12:30 TU
The violent clashes are reported to have occurred at the central Haft-e-Tir square when riot police waded into a crowd of several hundred opposition supporters who were chanting "Death to the dictator".
Opposition supporters have since June been staging protests at every opportunity in Tehran against the re-election of Mahmoud Ahmadinejad in a presidential vote they claim was massively rigged.
The protesters refused to disperse, leading to dozens being beaten or arrested.
Groups of pro-government hardliners also gathered at the square chanting "Death to America."
According to the opposition website Mowzcamp.com, the opposition leader Mehdi Karroubi was among the protesters at the square.
Other demonstrators were to be found chanting in the streets, changing location swiftly when police came to disperse them.
Witnesses said the entire city centre had become a stage for "cat and mouse games" between police on bikes and young protesters.
Thousands of Iranians also gathered outside the old US Embassy complex to mark the 30th anniversary of the capture by radical Islamist students of the compound, just months after the Islamic revolution toppled the US-backed shah. The students took 52 American diplomats hostage and held them for 444 days.
On Wedensday, the demonstrators also smashed up posters they had brought with them of the American Uncle Sam symbol and chanted "The blood in our veins is a gift to our leader" - a reference to Iranian supreme leader Ayatollah Ali Khamenei.
US President Barack Obama, meanwhile, in a statement urged Iran to look to the future rather than the past.
"We have heard for 30 years what the Iranian government is against; the question, now, is what kind of future it is for," he said.
"It is time for the Iranian government to decide whether it wants to focus on the past, or whether it will make the choices that will open the door to greater opportunity, prosperity and justice for its people."