Police in Serbia's capital, Belgrade, have used tear gas to disperse crowds protesting at alleged fraud in last week's general election.
They intervened after some of the crowd outside the city hall smashed windows and attempted to force their way in.
Opposition activists say police used excessive force on Sunday.
They say the government manipulated the elections won by the ruling party. President Aleksandar Vucic says the allegations are "rubbish and lies".
The Green-Left Front's co-leader, Radomir Lazovic, says officers beat him and a number of other people with truncheons.
There have been peaceful protests every evening since last week's national and local elections. Sunday's were the first to turn violent.
Supporters of the Serbia Against Violence opposition coalition attempted to break into the city hall.
Srdjan Milivojevic and Vladimir Obradovic, two opposition leaders, tried to open the door to the building. Police used pepper spray to repel the protesters.
Opposition leaders suspect agents provocateurs were involved in smashing windows, triggering the police intervention.
Sofija Mandic, a lawyer, told those assembled that Mr Vucic should "hand over power peacefully". The government was "usurping" power, she added.
The protesters say the government bused in thousands of people to swing the vote in Belgrade's city elections. They want the poll to be re-run.
The vote saw Mr Vucic's Serbian Progressive Party returned to power with a majority in parliament.
Domestic and international observers have reported "irregularities". There have been claims of ballot box stuffing and vote buying.
In an emergency address to the nation on Sunday evening, Mr Vucic blamed the unrest on foreign interference.
"Those who swore to fight against violence just confirmed once again that they are the real thugs," Mr Vucic said.
The opposition had hoped to deny the ruling party a majority and win control of Belgrade. But official results - immediately denounced as fraudulent by opposition figures - showed the opposition had failed to achieve either goal.
In response, it has mounted daily protests. Seven opposition figures have gone on hunger strike.
"Vucic stole thousands of votes," Marinika Tepic, one of the hunger strikers, said at Sunday's protest.
The electoral commission announced on Wednesday that the election would be re-run in some 30 polling stations, out of 8,000 nationwide.
Germany said allegations of electoral misconduct were "unacceptable" for a country hoping to join the EU.