Thousands of opposition supporters rallied peacefully in the Serbian capital Belgrade on Tuesday in an anti-government protest to demand the annulment of last month’s elections, which were described by international observers as unfair.

The populist ruling Serbian Progressive Party (SNS) won 46.72% of the votes in the snap parliamentary and local elections. But an international monitoring mission said the SNS gained an unfair advantage through media bias, the improper influence of President Aleksandar Vucic, and voting irregularities such as vote-buying.

Vucic and his allies said the elections were fair.

The rally was also staged to mark the sixth anniversary of the killing of Oliver Ivanovic, a moderate Kosovo Serb politician. Ivanovic was gunned down in front of his party office in 2018 in the ethnically divided town of Mitrovica, a predominantly Serb area of mainly Albanian Kosovo, which declared independence from Belgrade in 2008.

The investigation over Ivanovic’s death is still ongoing in both Serbia and Kosovo. In an address to the crowd, Marinika Tepic, a leader of the center-right alliance Serbia Against Violence (SPN), which came second in the vote, said the opposition remains determined to contest the election results.

“The entire planet knows, we caught him (Vucic) red-handed…him who would not dare to appear before his people,” said Tepic, who last month went on a two-week hunger strike to protest the outcome of the vote.

The opposition says it also wants the annulment of elections in Belgrade which ended inconclusively and are seen by many as fraudulent.

On Monday, Milos Vucevic, the head of SNS, said voting in Belgrade may be repeated if no one manages to form a coalition and secure a majority in the city hall.

With its population of 1.4 million people, Belgrade represents about a quarter of Serbia’s electorate, and its mayor is seen as one of the most influential officials in the Balkan country.

Serbia is a candidate for membership in the European Union, but before it joins the bloc, it must first reform the judiciary, rule of law, and media freedoms, and root out corruption, organized crime, and corruption.

It must also mend ties with Kosovo and align its foreign policies with those of the EU, including the introduction of sanctions against Russia, over its invasion of Ukraine.