With the angry crowd throwing stones at the UN police, it all escalated into a riot with gunfire and explosions directed at UN police officers

UN police spokesman Barry Fletcher

The police say they were shot at and had two grenades thrown at them - they returned fire and arrested a local Serb leader. His release is now being demanded by a self-styled security group known locally as the Bridge Watchers.

A BBC correspondent in Mitrovica, Nicholas Wood, says there is now a tense standoff in the city after one of the most violent days it has seen in a year.

Stone throwing

He says that since the end of the war in Kosovo in 1999, the UN has struggled to establish its authority in the north of Mitrovica where its presence is resented by the local Serbian population.

K-For has struggled to establish its authority in Mitrovica

John Neil, the head of the UN police in Mitrovica, said a crowd had formed in the afternoon when UN police set up a routine vehicle checkpoint.

He said they began throwing stones at the officers and Slavoljub Jovic-Pagi, a leader of a hardline Serbian group known as the Bridge Watchers, was arrested because he was throwing stones and giving instructions to others to do so.

"With the angry crowd throwing stones at the UN police, it all escalated into a riot with gunfire and explosions directed at UN police officers," UN police spokesman Barry Fletcher said.

'Civilians shot'

But Marko Jaksic, a leader in the Mitrovica Serb community, said the rioting only began after the UN police arrested Mr Jovic-Pagi.

Mr Jaksic said two civilians had been injured by bullets "apparently fired by riot police". He said that one of them had been shot in the neck and was in a critical condition.

Teams of Polish and Pakistani riot police have been deployed to the region.

The Bridge Watchers clashed with the UN police on Sunday. Crowds threw stones and surrounded police officers, after they attempted to stop what was thought to be a stolen car.

Our correspondent says it is over a year since Mitrovica has seen violence on this scale

The city has been divided between the Serbian dominated north and Albanian south, since the end of the war in Kosovo in 1999.

Ever since then the United Nations has struggled to establish its authority in the north, where the UN presence is resented by the local population.