(Bloomberg) -- Riot police in Serbia’s capital used pepper spray as they repelled a few thousand protesters who tried to storm Belgrade city hall amid a dispute over recent elections that opposition parties and European observers say were marred by irregularities.

President Aleksandar Vucic’s dominant Progressive Party won the Dec. 17 vote for the parliament. But in a much tighter race the same day for control over the capital, neither the president’s ruling party nor the main challenger, a coalition named Serbia Against Violence, gained a majority. The opposition demanded annulment of all results due to reports of vote-buying and unfair election conditions.

Opposition-led protests over the last week lapsed into violence on Sunday as a group smashed the doors of Belgrade City Hall but were stopped by the police. 

Two policemen sustained serious injuries while 35 protesters were arrested for a “violent attempt to seize institutions,” Vucic said early Monday, after chairing an urgent meeting of the National Security Council.

“The fight will continue,” opposition lawmaker Radomir Lazovic told regional broadcaster N1, saying police had used excessive force to disperse the crowd. He accused Vucic of being a “liar, a manipulator and a thief” who wouldn’t accept that his party had lost the local ballot in Belgrade.

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“No revolution is in progress and they will not succeed in anything they’re doing,” Vucic said in a televised, ad-hoc address to the Balkan nation. He called for calm and said the protest is a “product of much more serious geopolitical circumstances.”

Rejecting the allegations of rigged elections, the president vowed to defend Serbia’s “independence and sovereignty” against any threats, including “the quasi-politicians who will be held responsible for their violent behavior.” 

(Updates with arrests, injuries, opposition comment, from fourth paragraph)

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