Several thousand people gathered in front of the central election commission building in Belgrade on Monday to protest over an election earlier this month.
The protesters marched to the main police station where they believed those detained by police were being held.
Earlier in the day, police said 38 people had been detained during and after a protest over election results on Sunday. The police said eight policemen were injured in clashes.
Protesters broke windows and glass at the main entrance of the town hall, before police used pepper spray to disperse them around 10 pm on Sunday (1 am on Monday, UAE time).
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 relation to protests last night," Ivkovic told a news conference.
The opposition parties accused police of excessive force.
According to state election commission preliminary results, the populist ruling Serbian Progressive Party (SNS) won 46.72% of the votes in the December 17 snap parliamentary election.
Serbia Against Violence came second in the election with 23.56% of the vote, and the Socialist Party of Serbia third with 6.56%.
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