BELGRADE, Serbia >> Serbian police said Monday they detained at least 38 people who took part in a protest against reported widespread irregularities during a recent general ballot that declared the governing populists as winners of parliamentary and local elections.

Opposition group Serbia Against Violence has been staging protests since the Dec. 17 elections, saying there was election fraud, particularly in the capital, Belgrade. Some opposition politicians went on hunger strike.

On Sunday evening, protesters tried to enter Belgrade city hall, breaking windows, before riot police pushed them back using tear gas, pepper spray and batons.

More protests were held on Monday evening. Hundreds gathered outside the seat of the state election commission in central Belgrade before walking toward city police headquarters, where the detained opposition supporters are believed to be held.

Representatives of Serbia Against Violence have said they will not recognize the election results, calling instead for the vote to be annulled and held again in a free and fair election. The alliance last week urged the EU to investigate the results after international observers said the vote was held in “unjust conditions.”

U.S. Ambassador Christopher Hill, who has been criticized by the opposition for his alleged pro-government statements, on Monday denounced “violence and vandalism” during Sunday’s protest. On X, formerly Twitter, Hill added that “the legitimacy of democratic processes depends upon transparency and on the readiness of all parties, winning or losing, to respect the will of the people as expressed at the ballot box.”

Senior police official Ivica Ivkovic told reporters that those detained were facing charges of inciting violent change to the constitutional order — in reference to attempting to overthrow the government — and violent behavior. He added that eight officers were injured, including some seriously.

The opposition said that police used excessive force and beat up some of its supporters.

Also on Monday, several hundred university students and other citizens blocked traffic at a key Belgrade street that hosts government headquarters, braving a police warning against blockades in the capital. No incidents were reported as some students played volleyball and soccer on a sunny day.

Police “are ready and capable of countering any acts of violence with determination,” Ivkovic said.

The Serbian Progressive Party, which won the election, has denied rigging the vote and insisted the ballots were fair despite criticism from international monitors and local election observers.

President Aleksandar Vučić described Sunday’s protests as an attempt to overthrow the government with help from abroad, without specifying what he meant. Serbia’s Prime Minister Ana Brnabić thanked Russia late Sunday for tipping off Serbia in advance of violent protests against the results.

Vučić has “irrefutable evidence” that the West is encouraging the opposition protests, Russian Ambassador Aleksandr Botsan-Kharchenko told Russian media after meeting with Vučić on Monday.

In a post on Instagram on Monday, Vučić also referred to “mentors from abroad” of his political opponents leading the protests but offered no other details. Police officers and state property were “brutally” attacked by those who want to demolish democracy and the election will of Serbia’s citizens, Vučić said.

Serbia is formally seeking membership in the European Union, but the Balkan nation has maintained close ties with Moscow and has refused to join Western sanctions imposed on Russia over the full-scale invasion of Ukraine.

Vučić’s party claimed victory in both the parliamentary and Belgrade city ballots elections. Serbia Against Violence, the governing party’s main contender, said that it was robbed of a win in Belgrade.

Representatives of several international rights watchdogs observing the elections reported multiple irregularities during the vote, including cases of bought votes and ballot-box stuffing. They also noted unjust conditions for opposition candidates because of media bias, abuse of public resources by the ruling party, and Vučić dominating the ruling party’s campaign and media time allocated for candidates despite not taking part in the elections himself.

Dusan Stojanovic contributed to this story.