Serbia's President Aleksandar Vucic called for calm after riot police fired pepper spray at anti-government protesters who demanded the annulment of parliamentary and local elections a week ago, which international observers said were unfair.
The populist ruling Serbian Progressive Party (SNS) won 46.72 per cent of the votes in snap parliamentary elections last weekend, according to state election commission preliminary results.
An international monitoring mission on Monday said the SNS gained an unfair advantage through media bias, the improper influence of Mr Vucic and irregularities such as vote buying.
On Sunday, police fired pepper spray after a crowd tried to break into the Belgrade town hall where the country's local election commission is based.
Some of the protesters climbed the building and broke windows.
"Vucic thief," protesters chanted.
Eight policemen were injured and 38 people detained during and after the opposition protest over election results, Serbia's police said on Monday.
Mr Vucic said the disruption was an attempt orchestrated from abroad to overthrow the government. He said more than 35 people were arrested and more detentions would follow.
“No revolution is in progress and they will not succeed in anything they’re doing,” Mr Vucic said in a televised, ad hoc address to the Balkan nation.
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," Mr Ivkovic told a news briefing.
Serbia's ally Russia said foreign-backed forces were trying to foment trouble.
"There are processes and attempts by third forces, including from abroad, to provoke such unrest in Belgrade," Kremlin spokesman Dmitry Peskov said.
"We have no doubt that the leadership of the republic will ensure the rule of law."
Mr Peskov called for calm and said the protest is a “product of much more serious geopolitical circumstances”.
In a statement, the Interior Ministry called on protesters to "refrain from breaking into the town hall".
"By reacting calmly we are trying not to hurt protesters," Mr Vucic said in his address in the early evening.
The centre-left opposition alliance Serbia Against Violence came second in the election with 23.56 per cent of the vote, and the Socialist Party of Serbia third with 6.56 per cent.
Srdjan Milivojevic and Vladimir Obradovic of the Serbia Against Violence coalition tried to open the door of the town hall but were unable to enter, while the crowd shouted "get in, get in," and "no surrender".
Another member of Serbia Against Violence, Marinika Tepic, has been on hunger strike since the elections to demand they should be annulled.
