Riot troops serving with the Nato-led peacekeeping force, K-for, used stun grenades and tear gas to disperse the crowds. At least 18 people were treated for injuries.

The violence follows the killing of a 15-year-old Alban ian boy in a grenade attack in the Serb dominated north of the city on Monday.

UN officials are braced for more trouble in the run-up to the anniversary on Saturday of the murder of six Albanians in their homes by gangs of Serbs.

The protest started with a peaceful demonstration by students outside the city's municipal headquarters. As more people gathered, the crowd broke through flimsy barriers and ran past French troops towards the bridge over the River Ibar, which divides Mitrovice.

Some youths threw rocks and soldiers responded by firing tear gas and eventually using stun grenades. Two armoured vehicles were ransacked and set alight. A crowd of Serbs gathered on the other side of the river taunted the Albanians, but most of the anger was aimed at the French.

"The revolt is directed against French K-for because they don't respect international codes of human rights. All the crimes in the north have been committed in their presence, and often with their aid," said Milzim Krazniqi, a 40-year-old protester.

Away from the bridge, a French military base in a former hotel in the south of the city came under a hail of stones. Soldiers drew up in a line of armoured personnel carriers and fired stun grenades in response.

A spokesman for the UN condemned the violence, saying that protesters had refused to heed calls for calm. The head of the UN administration in Mitrovice, Tony Welch, said the cycle of violence was prompted by groups of Serbs in the north of the city intent on "testing the will of the new K-for brigade" who arrived two weeks ago.

He was referring to an incident on Saturday in which a group of Serbs shot at French troops as they intervened to protect two Albanians from being attacked. The attackers' car was traced by K-for to the Serbian national council, home of Mitrovice's Serb leadership. The building was later searched by K-for.