Thousands of Serbs chanting "Kosovo is Serbia" marched Tuesday to a bridge dividing them from ethnic Albanians while others torched U.N. border checkpoints and cars to protest Kosovo's declaration of independence.
NATO troops later closed down the roads leading to the checkpoints, cutting off the only link between northern Kosovo and Serbia, said Besim Hoti, a U.N. spokesman.
The move appeared to be due to fears that loss of U.N. control of the border could allow Serbian militants to return to fight in Kosovo, a land Serb nationalists consider the cradle of their state and religion.
The divided town of Kosovska Mitrovica in northern Kosovo has been tense since the majority ethnic Albanian leadership in Pristina unilaterally declared independence from Serbia on Sunday - widely expected after internationally mediated talks on the province's future fell apart last year.
For two days, Kosovo's minority Serbs have shown their determination to shun the declaration by destroying U.N. and NATO property, setting off small bombs and staging noisy rallies.
The top U.N. official in Kosovo, Joachim Ruecker, condemned the attacks. "Any violence is completely unacceptable and will not be tolerated," the German diplomat said.
Kosovo has not been under Serbia's control since 1999, when NATO airstrikes halted a Serbian crackdown on ethnic Albanian separatists. A U.N. mission has governed Kosovo since, with more than 16,000 NATO troops and a multiethnic police force policing the province.