Police in helmets and wielding clubs fired at least seven rounds of tear gas and arrested up to 60 people at a rally in Haft-e Tir square, a popular shopping destination in the heart of Tehran, a witness said.
The Guards - an elite force set up in the wake of the 1979 revolution - threatened a "decisive and revolutionary" riposte to any further unrest as the Islamic regime battles to contain an escalating crisis over the election.
The warning came after state radio said at least 457 people had been detained in street clashes in Tehran on Saturday that left at least 13 dead.
The opposition leader Mir Hossein Mousavi, who has led a wave of massive protests over what he says was a rigged election that returned Mahmoud Ahmadinejad to power, urged supporters to continue demonstrating but to adopt "self-restraint" to avoid more bloodshed.
Witnesses said that helicopters hovered overhead as the protesters gathered at Haft-e-Tir Square. But hundreds of anti-riot police quickly put an end to the demonstration and prevented any gathering, even of small groups, at the scene.
At a nearby subway station, police did not allow anyone to stand still, asking them to keep on walking and separating those who walked together.
The abortive protest came after the state election watchdog admitted that in 50 cities the number of votes cast in this month presidential election exceeded the number of eligible voters.
The surprising admission by the Guardian Council was, however, designed to undermine the claims of the defeated candidates that the vote was rigged.
Mr Mousavi, President Mahmoud Ahmadinejad's main rival in the hotly-disputed election, and the other two losing candidates have claimed that the vote exceeded eligible voters in as many as 170 districts.
Abbasali Kadkhodai, a spokesman for the council of senior clerics, told the state television channel IRIB: "Our investigation shows that the number of districts they announced is not correct. Based on our preliminary report, 50 districts face this issue."
Mr Kadkhodaei also argued that voter turnouts of more than 100 per cent were not unusual because Iranians can cast their ballots where they want. Although it is summer in Iran and some of the cities in question are in desert areas, he suggested some voters might have gone to them on holiday.