BELGRADE, SERBIA -- Serbia's ruling populists insisted on Tuesday that weekend snap elections were free and fair despite criticism from international observers who noted multiple irregularities and unjust conditions for parties during the vote.

Political tensions in Serbia soared over the snap parliamentary and local election on Sunday. Several thousand people rallied on Monday to protest alleged fraud at the ballot for municipal authorities in Belgrade, the capital. More protests are planned Tuesday.

Early results showed victory of President Aleksandar Vucic's Serbian Progressive Party in both ballots. But its main opponents, the Serbia Against Violence alliance, say they were robbed of a win in Belgrade.

Opposition leaders said they will not recognize the result in Belgrade, called protests and demanded that the vote be annulled and held again. The populists rejected the claims.

"We are very happy how the election day went," Milos Vucevic, the leader of the right-wing Serbian Progressive Party, said on pro-government TV Prva on Tuesday morning. "It (election) can set an example for many other countries."

In a preliminary statement, a mission made up of representatives of international rights watchdogs said on Monday that the Serbia vote was "marred by harsh rhetoric, bias in the media, pressure on public sector employees and misuse of public resources."

Serious irregularities also included cases of vote-buying and ballot box stuffing, according to the joint conclusions by the OSCE Office for Democratic Institutions and Human Rights, the OSCE Parliamentary Assembly, the Parliamentary Assembly of the Council of Europe and the European Parliament.

Germany's Foreign Ministry later on Monday wrote on X, formerly Twitter, that "Serbia has voted but the OSCE ... is reporting abuse of public funds, intimidation of voters and cases of vote-buying."

"This is unacceptable for a country with EU candidate status," the statement said.

The Kremlin congratulated Vucic on the election victory, calling Serbia a "brotherly" and "friendly" nation.

Vucic, who has been in power since 2012, has dismissed criticism from his opponents that his government curbed democratic freedoms while allowing corruption and organized crime to run rampant.

Under Vucic, Serbia became a candidate for EU membership, but the opposition accuses the bloc of turning a blind eye to the country's democratic shortcomings in return for stability in the Balkan region, still troubled after the wars of the 1990s.

In Belgrade, several thousand people chanted "thieves" in front of the state election commission headquarters on Monday evening. Some opposition politicians spent the night in the building after lodging formal complaints.

Serbia Against Violence includes parties that were behind months of street protests this year triggered by two back-to-back mass shootings in May. The group has charged that some 40,000 people were bused in from neighbouring Bosnia to vote in Belgrade and tilt the outcome in favour of the populists.

Skirmishes erupted during Monday's rally as mostly young protesters pushed against the metal fences and shoved a commission member. Two people, aged 19 and 20, have been detained for violent behavior, police said Tuesday.

Danica Samardzic, a student from Belgrade, said she came to the protest because "we have been robbed."

"We want something to be done about all the problems we have in our country," she said, adding that "I was literally crying" during president Vucic's victory speech. "We should not be silent about this, this protest is just the beginning of something bigger."

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Associated Press writers Dusan Stojanovic in Belgrade and Geir Moulson in Berlin contributed to this report.