An ethnic Albanian teenager has been arrested over the attack in the town, scene of some of the worst clashes between ethnic Albanians and Serbs since a 1998-99 war in the UN-administered Serbian province, a police source told Reuters.
After the attack, hundreds of Serbs gathered on the main bridge spanning the river that divides Mitrovica's Serbs and ethnic Albanians. The rundown former mining town is patrolled by French NATO soldiers and 500 UN police officers.
Larry Miller, the UN police spokesman, said: "Someone passing by threw an explosive device at the café."
Hospital officials said two Serbs had been seriously wounded in the attack. A British UN police officer was also caught in the blast but doctors said he was not badly hurt.
Legally part of Serbia, Kosovo has been run by the United Nations since 1999 when a NATO military campaign drove out Serb forces accused of ethnic cleansing in the war.
The 90% Albanian majority is pushing for independence in talks that began in February but tensions are rising as the United Nations nears a decision on Kosovo's so-called "final status".
Martti Ahtisaari, the UN chief mediator, gave no hint of a breakthrough in the talks during a visit this week to Kosovo's capital Pristina.
Diplomats say independence is the most likely outcome but Serbia shows no sign of conceding to the loss of the southern province, still home to about 100,000 Serbs and scores of centuries-old Serb Orthodox churches.
About 10,000 Albanians were killed in the war. At least half the Serb population fled a wave of revenge attacks after Serb troops withdrew from the province.
NATO has a 17,000-strong peacekeeping force in Kosovo.