Hundreds of Kosovo Serbs have taken over a UN courthouse in the northern Kosovo town of Mitrovica, raising a Serbian flag to replace the UN's.
Kosovo Serbs are refusing to recognise Pristina's authority
The protesters broke into the building in the Serb-dominated part of the city, forcing UN police to retreat.
A Kosovo police spokesman said a UN official was negotiating with Serb leaders to try to resolve the issue.
Kosovo Serbs and Serbia have refused to recognise Kosovo's declaration of independence last month.
Most EU states and the US have recognised Pristina's unilateral move.
UN riot police did not intervene when the crowd seized the court.
"We have returned to a building that belongs to us, and in which we worked until 1999," municipal public prosecutor Milan Bigovic was quoted as saying by Reuters.
Many of the protesters are reported to be former staff who lost their jobs in 1999 at the end of the war in Kosovo, when it came under UN administration.
Serbs had staged rallies outside the court for several weeks, preventing ethnic Albanian court employees from crossing the bridge over the Ibar River that divides Mitrovica into a Serb-run north and an ethnic Albanian south.
Tension in the region has risen sharply since Kosovo declared independence on 17 February.
Last week, Serbs tried to take control over a railway line in the northern Kosovo.
In February, some 150 Kosovo Serb police officers were suspended for refusing to take orders from the ethnic Albanian authorities in Kosovo's capital, Pristina.