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.