A Reuters reporter saw Polish special police units, part of a European Union mission, open fire with rubber bullets, during one of the worst bouts of civil unrest since Kosovo declared independence from Serbia in 2008.
The violence broke out when several hundred Albanians, protesting over the Serbs' closure of the bridge for the past three years, began hurling rocks and bottles at Kosovo police.
They set fire to cars belonging to the officers and the EU law and order mission. Kosovo police responded with teargas.
US soldiers, part of a NATO peace force of some 5,000, guarded the bridge in four rows of Humvees.
A police spokesman said 13 police officers and 10 civilians were wounded. Five people were arrested. By evening, the situation was calm but NATO troops, EU and Kosovo police were holding their positions.
President Atifete Jahjaga said: "Any attempt to find a solution through the use of violence will further escalate the situation and will be a step backwards."
Mitrovica has been a frequent flashpoint between Serbs and Albanians since Kosovo's 1998-99 war, when NATO intervened with 11 weeks of air strikes to halt the massacre and expulsion of Albanians by Serbian forces waging a counter-insurgency war.
Serbia does not recognise Kosovo as independent, but agreed to give up de facto control over a small Serb pocket of northern Kosovo last year under a deal brokered by the EU. In exchange, Belgrade won the green light to open talks on joining the EU.
But Serbs in the north are reluctant to integrate with Kosovo's ethnic Albanian majority.
They blocked the bridge in 2011 following an abortive bid by the Kosovo government to rein in the north. The Serbs dismantled the roadblock on Wednesday, only to replace it with a "Park of Peace" consisting of concrete plant pots and earth.
