MITROVICA, Serbia, March 27 (Reuters) - A hand grenade blast damaged three cars and property in a tense, ethnically mixed pocket of northern Kosovo on Tuesday, police said. It was the second hand grenade explosion this week in the Bosnjacka Mahala, a mainly ethnic Albanian community that sits isolated on the Serb side of the divided city of Mitrovica. No one was hurt. "An explosive device detonated, damaging three cars and some shop windows," United Nations police spokesman Larry Miller said. A second police source said the device was a hand grenade, which exploded outside a Serb-run cafe. U.N. police and NATO peacekeepers in the province are braced for possible violence after a U.N. envoy on Monday submitted a plan for independence, eight years after NATO bombs drove out Serb forces accused of atrocities in a war with guerrillas. Kosovo has been run by the United Nations since. Mitrovica, which has witnessed some of the worst clashes since the 1998-99 war, is seen as a likely flashpoint between Serbs in the north and Albanians across the river in the south.