Witnesses said police, some in plain clothes, used batons in street fights with the protesters, who jeered as they left the demonstration past cordons of riot police. At least one person was beaten unconscious, and a man who said he was walking home from a medical conference received an open head-wound, eyewitnesses said.
A mother and her 12-year-old daughter were slightly injured, and two foreign television crews had their cameras smashed. A reporter from the independent Radio Index said: "It seems riot police are intervening in response to jeering. No other provocation took place."
The former Spanish prime minister, Felipe Gonzalez, representing the Organisation for Co-operation and Security in Europe (OSCE), confirmed opposition victories over Slobodan Milosevic and his ruling Socialist party (SPS) in Belgrade and 14 other towns.
Soon after Mr Gonzalez announced his verdict in Geneva, the news spread to Belgrade and supporters of the opposition Zajedno (Together) coalition who were participating in the 38th day of protest marches.
The crowd of some 80,000 danced and cheered Mr Gonzalez's report, which added weight to mounting international demands that President Milosevic respect democracy.
Mr Gonzalez, who headed an OSCE mission to Belgrade this month, called on the 53-nation grouping to issue an "urgent appeal" to the authorities and political forces in Yugoslavia to "comply with the will expressed at the polls by the citizens". He said the authorities should accept opposition victories in 22 disputed municipalities, including nine in Belgrade.
Opposition leaders were sceptical that Mr Milosevic would respond to the Gonzalez report. "We do not expect him to accept the findings of this commission. We expect further escalation and aggravation of this political crisis," said Zoran Djindjic, leader of the opposition Democratic Party.
The official Tanjug news agency reported that Mr Gonzalez confirmed the SPS and its allies won a majority of the votes but did not refer to his endorsement of Zajedno victories.
Weeks of processions through the centre of Belgrade ended on Thursday when riot police cleared the streets and forced demonstrators back into a pedestrian square.
Despite international warnings to allow the marches to continue, the authorities also curtailed yesterday's march by the opposition coalition.
The Yugoslav Foreign Minister, Milan Milutinovic, said the OSCE had delivered a "balanced" report on the elections, but had confused some facts when it visited Belgrade.
Mr Milutinovic said the most important feature of the report was that it called for dialogue "within the framework of the system" and "discussion between the authorities and the opposition in parliament".