(CNN) -- Violent clashes with Serbs who took over a U.N. building in northern Kosovo led to dozens of injuries and arrests and forced the United Nations to withdraw its police personnel from the area.
About 40 U.N. police and at least 12 NATO soldiers were hurt in the violence Monday morning, U.N. spokesman Alexander Ivanko said told CNN.
A local journalist said about 20 others, mostly protesters, were taken to a hospital for treatment of shock, and a Serb member of the Kosovo police force was being treated for a gunshot wound.
The clashes began when the U.N. and NATO forces started an operation early Monday to evict about 300 Serb protesters who had taken over a U.N. court building in north Mitrovica, a divided Kosovo town where ethnic Serbs control the northern half and ethnic Albanians control the southern half.
The Serbs took control of the building Friday and the United Nations tried over the weekend to negotiate with them to leave, Ivanko said.
When there was no resolution by Monday, U.N. police decided to use force, he said. The Serbs had been told very clearly as early as Sunday night that the U.N. forces would move in to take control, he said.
"There are reports of possible gunfire and some grenades used against U.N. police and KFOR," Ivanko said from Pristina, the capital of Kosovo , as the violence flared. KFOR is the abbreviation for the NATO Kosovo Force.
Ivanko said the protesters also threw rocks, Molotov cocktails, and firecrackers at police.
NATO troops responded with tear gas, Ivanko said.
The violence prompted the United Nations to evacuate all international police personnel from northern Mitrovica.
"The reason for this is the very serious security situation," Ivanko said. "We do have a substantial number of police officers there but they're under constant attack, and that's why the police commissioner decided to hand over the dealing of this situation, as we speak, to KFOR."
U.N. police would likely continue controlling the entry points into Kosovo after withdrawal, Ivanko said, but KFOR troops will take over security.
The injured protesters were taken to the hospital for shock because police used stun grenades to control the crowds, according to a local journalist who asked not to be named.
Stun grenades emit a deafening noise when they explode but are not intended to injure.
The violence was some of the worst in Kosovo since it declared independence last month. Kosovo had been a Serbian province, and Serbia's government has strongly rejected the declaration. 
Watch Serbs protest against NATO tank. »
The United States, which is among the countries that has recognized Kosovo's independence, urged all sides in Kosovo to work together.
"It's important that Kosovo develop as a multi-ethnic society," State Department spokesman Tom Casey told CNN. "A plan is in place to ensure minority rights."
Regarding increased fears about the partition of Kosovo along ethnic lines, Casey said, "I don't think anyone believes that a partition of Kosovo is a remote possibility. Kosovo will be Kosovo now and in the future."
Ivanko said about 40 people were arrested in the clashes. They were arraigned in Pristina and were expected to be released later in the day, Ivanko said.
The journalist described the scene as quiet but tense by mid-morning, with KFOR troops patrolling the street.
The protesters who took over the U.N. building in Mitrovica were angry over losing their court jobs in 1999, Ivanko said. E-mail to a friend 
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