BELGRADE, Serbia (AP) — Riot police in Serbia fired tear gas to prevent hundreds of opposition supporters from entering the capital’s city council building on Sunday 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. Serbian President Aleksandar Vucic said Sunday that those claims were blatant ”lies” promoted by the political opposition.

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 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.

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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, an abuse of public resources and the president’s dominance during the campaign.

“Police are everywhere, also on the roofs. It is obvious that they do not want to recognize (the) election results. We will continue with our fight,” Nebojsa Zelenovic, one of the leaders of the alliance, said.

The vote has caused political tensions in Serbia, a troubled Balkan nation that is seeking close ties with Russia but also European Union membership.

Serbia Against Violence said in a letter sent Thursday to EU institutions, officials and member nations that it would not recognize the outcome of the elections.

The alliance called on the EU to do the same and to initiate an investigation.

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Associated Press writer Jovana Gec contributed to this story.