MITROVICA, Kosovo Dec 11 (Reuters) - Serb protesters in northern Kosovo blocked main roads for a second day on Sunday following a nighttime exchange of fire with police after the arrest of a former Serb policeman, amid rising tensions between authorities and Kosovo's Serb minority.EULEX, the European Union mission tasked with patrolling northern Kosovo, said a stun grenade was thrown on one of its armoured vehicles on Saturday evening, but no one was injured.He was part of a mass resignation of Serbs from the force last month, after Pristina said it would require Serbs to scrap Serbian license plates dating to before the 1998-99 Kosovo War that led to independence.EU PLAN IN DANGERKosovo declared independence from Serbia in 2008 with the backing of the West, following the 1998-99 war in which NATO intervened to protect Albanian-majority Kosovo.Serb mayors in northern Kosovo municipalities, along with local judges and some 600 police officers, resigned last month in protest over a government decision to replace Belgrade-issued car licence plates with ones issued by Pristina."