Cloaked in medieval costumes, a choir sang the national anthem of Serbia at a protest organized in the administrative center of the Serb-majority north of Kosovo as tensions intensify between ethnic Serbs and the Albanian majority in the country.

Hundreds of demonstrators gathered Sunday on the main thoroughfare in North Mitrovica around a podium erected for the purpose. Serbian nationalist songs were played and speeches were proclaimed.

The costumes worn by the choir harken back to the Serbian medieval kingdom, a time in history when Serbia had greater control of Kosovo and is often a source of symbolism for those who want to show their loyalty to Belgrade.

“We are gathered today to send a strong message to Pristina and the international community to call for an end to the subjugation and harassment of Serbs,” said Goran Rakić, the leader of Srpska Lista, the main ethnic Serb party in Kosovo.

This comes on the heels of the first collective resignation of Kosovo Serbs from the country’s institutions since 2013, when the slow integration of the ethnic Serb community was launched as a result of the ongoing dialogue between the former belligerents facilitated by the European Union.

Serbs withdrew from the parliament, the judiciary and the police in one fell swoop on Saturday, and Rakić resigned his position as Minister of Communities and Returnees. The mayors of the four municipalities with a Serb majority along the border between Kosovo and Serbia also resigned.

“The situation in the last couple of days in the north of Kosovo has been peaceful but tense, and people were unsure about what will happen next,” said Jovana Radosavljević, the head of the New Social Initiative NGO that is based in North Mitrovica.

Hostilities between the ethnic Serb majority in the north of the country and the government in Pristina began escalating on Wednesday after the head of regional police department refused to implement a directive to issue warnings to those who had not transferred to Kosovo-issued license plates.

The license plate issue has been a source of contention for over a year, with local Serbs preferring to use plates issued by Belgrade that are illegal in Kosovo. A handful of cars that had transitioned to the new plates have been burned in recent weeks, a continuation of flareups last witnessed over the summer.

“We cannot implement this agreement because it is a political one and primarily directed against the Serb community,” Nenad Đurić, the head of the Regional Directorate North, said in a statement on Wednesday, explaining that police traffic patrols were dispatched to the north from Pristina in order to implement this decision. He was subsequently suspended.

After the resignations, banners with Serbian flags and posters with the inscription “this is Serbia” appeared in North Mitrovica.

Kosovo’s Prime Minister Albin Kurti has said that he is not acting against the interests of the Serbs, but that “Belgrade is acting against me … my priority is respecting the constitution of the Republic of Kosovo.”

“Do not boycott and abandon the Kosovo institutions,” Kurti continued in a Facebook post. “Do not fall prey to political manipulations and geopolitical games.”

The EU has called for Kosovo Serbs to return to their positions in the country’s institutions.

The EU’s foreign policy chief, Josep Borrell, called on “both sides to refrain from any unilateral actions which might lead to further tensions.”

“Recent developments put years of hard work under Belgrade-Pristina Dialogue at risk,” Borrell said in a statement.

Serbia does not recognize the sovereignty of its former province, which declared independence in 2008. The oppression of the country’s ethnic Albanian population led to an armed conflict in the late 1990s that culminated with the NATO bombing of Serbia and Montenegro.

Local Serbs initially rejected the authority of Kosovo after the 1999 NATO bombing when Serb political, police and military forces withdrew from its former province, as well as its declaration of independence in 2008.

The relationship with Serbia and local Serbs has largely been guided by the EU-facilitated dialogue, which led to a series of agreements between the two governments and their representatives. Recently, there has been a Franco-German push for a final agreement to be signed between the two.

While no official version of the proposal has been published, EU officials pledged their support for their agreement at the Summit for the Western Balkans in Berlin this past week.