Eight policemen were injured and 38 people detained during and after an opposition protest over election results, Serbia's police said on Monday.
Thousands gathered in the center of Belgrade on Sunday to demand the annulment of parliamentary and local elections a week ago that international observers said were unfair.
The protesters broke windows and glass at the main entrance of the town hall, before police used pepper spray to disperse them around 10 p.m. (2100 GMT).
Police injured
Ivica Ivkovic, head of the police administration, said two of the eight wounded policemen sustained serious injuries.
"We will continue to work to maintain peace and order and we expect to see more arrests in relations with protests last night," Ivkovic told a news conference.
The opposition parties accused police of excessive force, and some social networks showed footage of policemen beating up men in streets near the town hall.
Outgoing Prime Minister Ana Brnabic thanked Russian secret service for providing information on planned activities by the opposition. "This (my statement) is not going to be popular in the West," Brnabic, of the ruling Serbian Progressive Party (SNS), said on a TV show.
A European Union aspirant, Serbia has resisted pressure by Western countries to introduce sanctions against Russia.
Moscow has been one of Serbia's closest allies for decades, especially after 1999 when Russia opposed the NATO airstrikes against rump Yugoslavia which comprised Serbia and Montenegro.
An international monitoring mission last Monday said the SNS gained an unfair advantage through media bias, the improper influence of President Aleksandar Vucic and voting irregularities such as vote buying.
The opposition led by the center-left alliance Serbia Against Violence said the protests will continue on Monday, with students planning to block traffic.
The populist ruling Serbian Progressive Party (SNS) won 46.72% of the votes in last weekend's snap parliamentary election, according to state election commission preliminary results.
Serbia Against Violence came second in the election with 23.56% of the vote, and the Socialist Party of Serbia third with 6.56%.
Kremlin says Serbian election protest is being stoked by foreign-backed forces
The Kremlin said on Monday that foreign-backed forces were trying to foment unrest in Serbia over parliamentary and local elections.
Thousands gathered in the center of Belgrade in an anti-government protest on Sunday to demand the annulment of elections a week ago that international observers said were unfair.