Serb protesters in northern Kosovo blocked major roads for a second day on Sunday after a nightmare of gunfire with police following the arrest of a former Serbian policeman amid rising tensions between the Serb minority and authorities.

In recent weeks, Serbs in northern Kosovo, a hotbed of Serbian nationalism, have met Pristina’s anti-Serb attempts with violent resistance.

EULEX, the European Union mission tasked with patrolling northern Kosovo, said one of the armored vehicle patrols was also attacked by protesters on Saturday night.

“A stun grenade was thrown at an EULEX reconnaissance patrol last night near Rudare,” the EU mission said in a statement. He said no one was injured.

The latest protests were sparked by the arrest of a former police officer on Saturday, who was part of an exodus of Serbs from the force last month after Pristina said she would enforce a law forcing Serbs to scrap old plates dating from before the 1998-99 guerrilla uprising that led to independence.

On Sunday, for a second day, trucks and other heavy vehicles blocked several major roads in northern Kosovo leading to two border crossings with Serbia. Both crossings were closed to traffic.

Late on Saturday, Kosovo police said they were attacked in different locations near a lake on the border with Serbia. The force said it had to return fire in self-defense and there were no immediate reports of injuries.

“The barricades of masked criminals in the north must be removed immediately,” Prime Minister Albin Kurti said in a statement, adding that his government was in contact with the NATO peacekeeping mission that has more than 3,000 troops on the ground.

Pristina police said former police officer Dejan Pantic was arrested Tuesday for allegedly attacking state offices, banging on the windows of election commission offices and police officers and election officials.

Serbian mayors of Kosovo’s northern municipalities, along with local judges and some 600 police officers, resigned last month in protest at the government’s decision to replace Belgrade-issued license plates with those issued by Pristina.

Serbian President Aleksandar Vucic said his country would ask NATO’s KFOR peacekeeping mission to allow Serbia to deploy soldiers and police in Kosovo, although he acknowledged there was no chance the permit would be granted.

“We do not seek conflict, but dialogue and peace. But let me be clear: the Republic of Kosovo will defend itself, with strength and determination,” Kurti said in response to Vucic’s comments.

Kosovo declared independence from Serbia in 2008 with Western backing following a 1998-1999 war in which NATO intervened to protect Kosovo, which has an Albanian majority.