Violence and low turnout in Kosovo elections underscored the deep divisions in the territory and dealt a blow to Belgrade, which had sought to further its EU membership bid by urging ethnic Serbs to vote peacefully.
The weekend vote, the first Belgrade has backed since ethnic-Albanian majority Kosovo proclaimed independence in 2008, was seen as a test of an April deal brokered by Brussels to normalize ties between the former foes.
Voting was interrupted late Sunday in the northern town of Kosovska Mitrovica, the main Serb-populated town in Kosovo, when masked Serb extremists stormed a polling station and destroyed ballot boxes, injuring a woman.
The European Union "condemn(s) the violent incidents in Mitrovica... which disrupted the otherwise orderly run electoral process in the rest of Kosovo," said Maja Kocijancic, spokeswoman for the bloc's foreign policy chief Catherine Ashton.
She said EU observers would deliver a "preliminary assessment" of the vote on Tuesday.
Tanja Miscevic, Serbia's chief negotiator with Brussels, said the incidents in Kosovska Mitrovica "are not a sign of democratic development."
"We are all aware of the importance of normalizing relations between Belgrade and Pristina," Miscevic told Tanjug news agency.
"Nobody wants to see ballot boxes destroyed and voters threatened in the 21st century," she added.
In Pristina, Kosovo's central election commission said it was yet to decide whether the vote should be annulled and rerun in the northern regions affected by the violence.
Hardliners in the Serb-run north reject any move integrating them with the rest of Kosovo and have campaigned for a boycott of the local elections.
Serbia also officially rejects Kosovo's freedom but it had encouraged the Serb minority to vote. This was to meet EU demands on ending de facto ethnic partition but also to gain more sway in Pristina institutions.
Some 120,000 ethnic Serbs live in Kosovo, whose 1.8 million population is mostly Albanian. Some 40,000 of them live in the north, where they make up the majority and enjoy control over some public institutions.
Sunday's incidents have "hindered a large part" of the April EU-brokered accord, said Milivoje Mihajlovic, spokesman for the Serbian government.
"All three sides, Pristina, Serbia and the international community have to seriously deal with this problem," Mihajlovic said.
In case the vote is repeated, Serbia "will have to send a clear message... that extremists and hooligans can not act unpunished" in Serb-populated areas of Kosovo, said Belgrade-based analyst Vladimir Pejic.
Belgrade immediately accused the boycott campaign of being behind the attacks and blamed Kosovo police and NATO-led peacekeepers (KFOR) for failing to prevent them.
Marko Jaksic, a pro-boycott Kosovo Serb politician, rejected the allegations and accused Belgrade of staging the violence to cover up the low turnout.
"We had no reason to disturb the vote as it went in accordance to our wishes," Jaksic told reporters.
The Organization for Security and Cooperation in Europe (OSCE), which had a key role in organizing the polls, said less than one in five Serb voters cast a ballot.
Provisional results showed a 47.9 percent voter turnout across other parts of Kosovo, with mayors elected in 10 municipalities.
Several runoffs will be needed but no date has been set.
Kosovo, the territory which sparked a war between Serb forces and ethnic Albanian rebels in 1998-1999, remains the main stumbling block to Serbia's bid to join the European Union.
The April deal with Pristina helped Serbia secure the green light to begin membership talks with Brussels, and holding up its end of the accord is vital for Belgrade.
For Kosovo, a free and fair vote with a significant Serb turnout would be a positive step in its own push for negotiations on an EU membership bid. Kosovo's independence is recognized by most EU states.