The Serbian army was put on full alert after Kosovo special police raided Mitrovica on Tuesday. Footage shows officers from the NATO-led Kosovo Force (KFOR) patrolling the streets in the city.
Local media in Kosovo reported that the raid in Mitrovica was an anti-smuggling operation against acting police officers and that it was met with armed resistance and anger in the Serb-dominated area.
Kosovo police say five police officers and six civilians were injured.
Two staff from the UN mission in Kosovo, including one Russian citizen, were also arrested but then released.
Serbian President Aleksandar Vucic told parliament in Belgrade that he ordered troops to be on alert and act if tensions escalate. He added that Serbia is interested in keeping the peace.
Serbia does not recognise the independence of Kosovo.