Serbia's populist President Aleksandar Vucic said it was an attempt to overthrow the government orchestrated from abroad. He said more than 35 people were arrested and more would follow.
"This was an attempted violent takeover of the state institutions of the Republic of Serbia," Vucic told the pro-government Pink TV.
The country's populist authorities have denied rigging the vote and described the election to fill parliament and local offices as fair.
Opposition supporters were pepper sprayed as they tried to storm city officers in Belgrade. (EPA PHOTO)
Vucic said Sunday that claims of irregularities were blatant "lies" promoted by the political opposition.
Vucic suggested the unrest was instigated from abroad but provided no evidence to support the claim. Addressing the nation on state media as the protest took place outside Belgrade city hall, he called the demonstrators "thugs" who would not succeed in destabilising the state.
"They will not succeed," 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 first barricaded themselves inside the city government building, firing tear gas and pepper spray as hundreds of opposition protesters broke windows at the entrance. Later, the police pushed the crowd from the downtown area and made several arrests.
The protesters shouted "open the door" and "thieves," as they pelted the building with eggs and stones.
Nebojsa Zelenovic, one of the leaders of the opposition Serbia Against Violence alliance, said police officers swarmed all of downtown Belgrade, including the roofs of buildings. The area is home to the national parliament and the presidential headquarters along with the city government.
There were no immediate reports of injuries.
Police made 35 arrests during the protest in Belgrade. (AP PHOTO)
Results from the December 17 election showed a victory for Vucic's Serbian Progressive Party in both the parliamentary and Belgrade city ballots.
Serbia Against Violence, the party's main opponent, said it was robbed of a win, especially in Belgrade.
"We will continue with our fight," Zelenovic said.
An observation mission made up of representatives of international rights watchdogs reported multiple irregularities, included cases of bought votes and the stuffing of ballot boxes.
The observers also noted unjust conditions for opposition candidates due to media bias, an abuse of public resources and the president's dominance during the campaign.
