Witnesses said several demonstrators - lobbing stones and firecrackers at police - were also injured.
Milan Ivanovic, the Mitrovica hospital director, said at least 100 people had been treated for the effects of tear gas.
More than 500 mainly Ukrainian UN police were involved in the dawn operation, Reuters news agency said.
A group of some 300 Serb protesters opposed to Kosovo's independence had captured the UN-run courthouse last Friday.
They refused to leave the premises after negotiations with United Nations officials failed over the weekend.
The Serbs - many of whom worked in Kosovo's judiciary before the territory came under the administration of the United Nations in 1999 - said they wanted to set up their own court.
Kosovo's government unilaterally declared independence from Serbia on February 17 and has since been recognised by many Western countries. Serbia and Kosovo Serbs vehemently reject the move as illegal.
Two days after the proclamation, angry Kosovo Serbs torched two border crossings with Serbia and have since staged a series of other protests, some of which have turned unruly.