Police in Serbia fired tear gas to prevent hundreds of opposition supporters from entering the capital’s city council building on Sunday.
The demonstrators were gathered in protest of what election observers said were widespread vote irregularities during a general election last weekend.
The country’s populist authorities have denied rigging the vote and described the election to fill parliament and local offices as fair.
President Aleksandar Vucic said on Sunday that those claims were blatant “lies” promoted by the political opposition.
Mr Vucic also suggested the unrest was instigated from abroad.
Addressing the nation during the protest outside Belgrade city hall, he called the demonstrators “thugs” who would not succeed in destabilising the state and said, “This is not a revolution”.
“They will not succeed,” Mr Vucic said.
“We are doing our best with our calm and mild reaction not to hurt demonstrators” who came to the event to protest peacefully.
Shielded riot police barricaded themselves inside Belgrade’s city hall, firing tear gas as hundreds of opposition protesters broke windows on the entrance.
The protesters shouted “Open the door” and “Thieves”. They also pelted the building with eggs.
Some chanted “Vucic is Putin,” comparing the Serbian president with Russia’s leader.
Later, the police pushed the crowd from the downtown area and made several arrests.
There were no immediate reports of injuries.
Results from the December 17 election showed victory for Mr Vucic’s Serbian Progressive Party in both the parliamentary and Belgrade city ballots.
The party’s main opponent, the Serbia Against Violence alliance, said it was robbed of a win, especially in Belgrade.
The vote has caused political tensions in the troubled Balkan nation, which is seeking close ties with Russia but also European Union membership.
In a letter sent to EU institutions, officials and member nations, Serbia Against Violence said it would not recognise the outcome of the elections.
The alliance called on the EU to do the same and to initiate an investigation.
