Clashes broke out on Sunday between Serbian police and opposition supporters threatening to storm Belgrade's city hall as they protested alleged irregularities in last weekend's elections.

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 detentions 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. Vucic said Sunday that claims of irregularities in the vote 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 destabilizing the state and said, "This is not a revolution."

"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. Some chanted "Vucic is Putin," comparing the Serbian president with Russia's leader.

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.

Results from the Dec. 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, abuse of public resources and the president's dominance during the campaign.