Some 50 people were reported injured in clashes between Kosovar police and visiting Serbs on their way to mark the anniversary of a historic battle.
The incident happened on Kosovo's border with Serbia as police forced out a group of some 70 Serbs carrying banners with nationalist messages.
The group was on its way to Gazimestan, where Serbs mark the 1389 Battle of Kosovo, which they lost to invading Ottoman Turks.
Kosovo's Interior Minister Bajram Rexhepi said some 30 officers were injured when the Serbs threw stones and other objects at police.
Belgrade media reported about 20 Serbs were injured, including one who was in a life-threatening condition and five others with gunshot wounds.
The UN condemned the flare-up in violence in Kosovo, and in particular an "attack on buses carrying a number of children of Serbian nationality."
The UN representative for Kosovo, Farid Zarif, called for calm and urged authorities to investigate Thursday's violence.
NATO also condemned the violence, and noted Molotov cocktails had been hurled at the buses.
Belgrade refuses to recognize the independence that majority ethnic Albanian Kosovo declared from Serbia in 2008.
Based on reporting by Reuters, AP, and B92