Serb protestors in Kosovo blocked roads for a second day on Sunday, following a nighttime exchange of fire with the police.
Kosovo's Serb minority is outraged by the arrest of a former Serb policeman suspected of involvement in attacks on Kosovo police.
Tensions in northern Kosovo, a hotbed of Serb nationalism, have mounted in recent weeks. Serbs have violently protested moves by authorities in Kosovo's capital, Pristina, which they deem anti-Serb.
A stun grenade was thrown at an armoured vehicle of EULEX, an EU mission tasked with patrolling northern Kosovo. No one was injured.
Josep Borrell, EU foreign policy chief, warned the bloc will not tolerate violence against members of its mission.
"Barricades must be removed immediately by groups of Kosovo Serbs. Calm must be restored," he tweeted on Sunday.
Hundreds of Serbs erected barricades on roads in the north of the country on Saturday, blocking traffic at two major border crossings with Serbia, police said. Both were closed on Sunday.
Trucks, ambulances and agricultural machinery were used to cut off traffic.
The latest protests were triggered by the arrest of a former police officer on Saturday.
He was one of two suspects wanted in connection with attacks on police patrols in recent days, according to authorities in Pristina.
Demonstrators told reporters they wanted to stop the arrested man from being transferred to Kosovo's capital.
Hostilities spiked after Pristina called for snap elections in four Serb-dominated communes in the north, organised for 18 December.
Shortly after the barricades were erected, Kosovo President Vjosa Osmani announced that she had decided to postpone local elections to April 23.
Serbs in the north reject Pristina's authority and Kosovo's independence from Belgrade. The main Serbian political parties have announced that will boycott any vote.
In November, there was a mass walkout of Serb policemen -- involving an estimated 600 officers -- amid an ongoing spat over vehicle licence plates.
The arrested police officer is reported to be one of those who resigned over Pristina's requirement that Serbs get rid of their Serbian license plates pre-dating the 1998-99 Kosovo War that led to independence.
Kosovo Prime Minister Albin Kurti has asked NATO's mission KFOR to remove the barricades.
"We call KFOR to guarantee the freedom of movement (and remove roadblocks) ... KFOR is asking for more time to finish this ... so we are waiting," Kurti said.
Late on Saturday Kosovo police said they came under fire in different locations close to a lake bordering Serbia. The force said it had to return fire in self-defence.
There were no immediate reports of injuries.
Serbia has given its ardent support to ethnic Serbs in Kosovo.
"There will be no surrender" in Kosovo, wrote Serbian President Aleksandar Vucic on his Instagram page. But he added Belgrade "will continue the fight with all legal means, for peace."
On Saturday, Vucic said Belgrade would ask KFOR to let Serbia deploy troops and police in Kosovo, although he acknowledged there was no chance of permission being granted.
"We do not seek conflict, but dialogue and peace. But let me be clear: the Republic of Kosovo will defend itself - forcefully and decisively," Kurti said in response to Vucic's comments.
The return of Belgrade's troops to the former Serbian province could dramatically increase tensions in the Balkans.
Kosovo declared independence from Serbia in 2008 with the backing of the West. This came after the 1998-99 war in which NATO intervened to protect Kosovo's Albanian majority.
The Serbian minority in Kosovo has a total of around 120,000 members.
It refuses to be loyal to authorities in Pristina. With the encouragement of Belgrade, it does not recognise Kosovo as an independent state.
Kosovo and Serbia are holding talks in Brussels to try to normalise relations and the EU has already presented a plan.