By Fatos Bytyci

ZVECAN, Kosovo (Reuters) -Serbian President Aleksandar Vucic placed the country's army on full combat alert and ordered army units to move closer to the border with Kosovo on Friday, after protesters and police clashed in a majority Serb town in the neighbouring country.

"An urgent movement (of troops) to the Kosovo border has been ordered," defence minister Milos Vucevic said in a live TV broadcast. "It is clear that the terror against the Serb community in Kosovo is happening," he said.

Police fired tear gas in the town of Zvecan to disperse a crowd from in front of a municipality building. The protesters were trying to prevent a newly-elected ethnic Albanian mayor from entering his office following an election which Kosovo Serbs had boycotted.

Some 50,000 Serbs living in four north Kosovo municipalities, including Zvecan, shunned the April 23 vote in protest that their demands for more autonomy had not been met - a new setback for a March peace deal between Kosovo and Serbia.

In a statement, the Kosovo police said five of its officers were slightly injured when protesters pelted them with rocks and other objects. Four police vehicles were attacked, including one that was set ablaze, the statement said. Gunfire was also heard in the area, it said.

Around 10 people sought medical attention in a local hospital for light injuries and the effects of tear gas, local Serb health authorities said.

Blerim Vela, chief of staff of Kosovo's President Vjosa Osmani accused "Serbia’s illegal and criminal structures" for escalating tensions and actions against law enforcement bodies.

"Violence will not prevail. Serbia bears full responsibility for the escalation," he said in a statement.

Several vehicles from the NATO peacekeeping mission in Kosovo have been seen in the vicinity of the site of the incident, while helicopters flew over the area, a Reuters reporter said.

Jeffrey Hovenier, the United States ambassador to Pristina condemned the police action.

"The U.S. condemns the ongoing action by Kosovan authorities to access municipal buildings in the north of Kosovo. Today's violent measures should be immediately terminated," he said on Twitter.

Chris Murphy, a U.S. Democratic senator and a member of the Foreign Relations Committee who recently visited Kosovo said he was "caught ... by surprise" by the incident.

"As a friend of Kosovo I am caught totally by surprise and he (Kurti) should end this provocation immediately," Murphy wrote on Twitter.

ELECTION BOYCOTT

The protests follow widely-boycotted local elections.

The election turnout was 3.47% and local Serbs said they would not work with the new mayors in the four municipalities - all from ethnic Albanian parties - because they do not represent them.

Earlier, police in the Kosovan capital of Pristina issued a statement saying that they were assisting the newly-elected mayors so they could enter municipal offices in the four northern municipalities.

The mayor in Zvecan was successfully escorted into his office, a Reuters reporter heard on a police radio.

Serbs in Kosovo's northern region do not accept Kosovo's 2008 declaration of independence from Serbia, almost a decade after the end of a war there, and still see Belgrade as their capital.

Ethnic Albanians form more than 90% of the population in Kosovo, with Serbs only the majority in the northern region.

The Western-backed plan verbally agreed to by the Kosovo and Serbian governments in March aimed to defuse tensions by granting local Serbs more autonomy, with the government in Pristina retaining ultimate authority.

(Reporting by Reuters TV, Fatos Bytyci, Aleksandar Vasovic and Ivana Sekularac; Editing by Frank Jack Daniel)