Serbian opposition demonstrators broke windows as they tried to storm Belgrade city hall on Sunday evening during protests over alleged election fraud, with police pushing them back with pepper spray, according to an AFP reporter on the scene.

Demonstrators used flagpoles, rocks, and eggs to break the windows of the capital's administrative building and tried to break in, but were quickly pushed back by police.

Around 10:00 p.m. (2100 GMT) local time, police dispersed the crowd.

Serbian President Aleksandar Vucic said that two police officers were "severely injured" during the protest and more sustained light injuries, while over 35 "bullies" were arrested.

Vucic described the incidents as an attempt at a "forcible takeover of state institutions", adding that there is evidence that "everything was prepared in advance".

"We have solid evidence to back it with and we will not give up … Nobody has the right to destroy our house, to destroy the property of our country and our citizens, not to mention causing serious injuries to our police officers," Vucic told pro-government Pink TV late on Sunday.

Serbia held parliamentary and local elections on 17 December, in which President Aleksandar Vucic's party said it had secured a commanding victory.

But a team of international observers — including representatives from the Organization for Security and Co-operation in Europe (OSCE) — report "irregularities", including "vote buying" and "ballot box stuffing".

The allegations sparked protests in front of Serbia's electoral commission building that lasted for days, and seven members of the main opposition camp — united under the banner "Serbia Against Violence" — began a hunger strike aiming at annulling the results.

Vucic said that the state "will be able" to arrest and bring to justice those responsible for the incidents in front of city hall.

"Scenes are dramatic because we are not used to someone breaking windows," Vucic told pro-government Pink TV in a special address, Beta news agency reported.

"But no revolution is taking place, and they won't succeed."

In a statement, Serbia's interior ministry urged the demonstrators to refrain from violence.

It added that the police had warned representatives of the main opposition camp before the elections that they had intelligence that violent demonstrations and breaking into the government buildings had been planned.

The police added that at that time opposition leaders had "guaranteed that no such events will happen", the statement said.