French Nato troops have fired warnings shots, hours after a Ukrainian policeman died from wounds suffered in clashes with Kosovo Serbs in Mitrovica.
Nato peacekeepers have taken up positions around Mitrovica
He was the seventh officer killed while serving with the United Nations mission since it began work in Kosovo in 1999.
More than 130 people were hurt in rioting after the UN took control of a courthouse seized by the Serbs.
Nato peacekeepers moved into the area and fired a stun grenade on Tuesday when Serbs began throwing stones.
The protesters were described as high-school students.
French and Spanish soldiers with the K-For force were stationed at the town's main flashpoints in an attempt to prevent a repeat of the previous day's violence.
Bridges over the Ibar river that divides the Serb north from the ethnic Albanian south were closed.
The Ukrainian officer who died had been part of a special unit which had moved in to the courthouse, arresting more than 50 Serbs.
He had suffered shrapnel wounds from a hand grenade.
There have been regular protests in Mitrovica since Kosovo declared independence from Serbia last month.
Kosovo Prime Minister Hashim Thaci told reporters: "We are very sad. The Kosovo institutions and government send condolences to the family and to his country."
The UN says 41 policemen are still being treated for their injuries.
Eleven French officers have been flown home for treatment. Ukrainians and Poles are also among the wounded.
Doctors say a Serb demonstrator who was shot in the head is in a coma, fighting for his life.
Nato admitted its troops had used "both lethal and non-lethal means" - but had only shot in the air.
Officials in Belgrade have accused the UN and Nato of using excessive force against demonstrators. Russia's UN Ambassador Vitaly Churkin also suggested there had been a lack of restraint.
Speaking in Moscow, US Secretary of State Condoleezza Rice called on all sides to refrain from violence and avoid "provocation".
On Tuesday, Japan became the latest country to recognise Kosovo's independence, joining the United States and a majority of countries in the European Union.