Macedonia held its general election two years early
General election voting was disrupted in five areas of Macedonia amid unrest in which at least one person was killed and more hurt.
There were several shootings, including two in the Albanian stronghold Aracinovo, where one person died when police clashed with gunmen.
The poll was called after Greece vetoed the former Yugoslav republic's attempt to join Nato because of its name.
Macedonia is also the name of a northern region of Greece.
The European Union's executive arm, the European Commission, said it was "very concerned" by the violence and called for "calm and restraint," while EU foreign policy chief Javier Solana demanded that polling be repeated in any affected areas.
A government spokesman said the reports of tainted polling stations affected just one percent of the 1.7 million electorate.
She said voting would be re-run wherever "violence, ballot rigging or other irregularities took place."
"It doesn't matter at all how much effort or resources will be needed. We will make sure it is all done properly, legally and, of course, with international monitoring," said government spokesman Ivica Bocevski.
'Assault on democracy'
Denis MacShane, the MP who is leading the UK observer delegation, called the violence and disruptions "an assault on democracy unacceptable in today's Europe".
He added: "No government can be formed as a result of this election. New polls must be organised in all the districts where violence, intimidation and stuffing of ballot boxes have taken place."
Prime Minister Nikola Gruevski called the election in April hoping to secure an absolute majority for his centre-right VMRO-DPMNE party.
The parliamentary election had originally been scheduled for 2010.
His party promises not to change the name of the country under Greek pressure, to push on with reforms aimed at EU admission, to revive the economy and to tackle corruption.
Greeks say the name implies a territorial claim over its northern province and resent a perceived attempt to claim the heritage of Philip of Macedon and Alexander the Great.
The country's ruling coalition is also divided over recognising Kosovo as an independent state.
Around a quarter of the population is ethnic Albanian, like most of Kosovo.
The next government is likely to face strong pressure from the ethnic Albanian parties to speed up the country's recognition of Kosovo, the BBC's Helen Fawkes reports.
Sunday's vote could determine whether Macedonia joins both Nato and the European Union, most of whose members did recognise Kosovo when it declared independence in February.
Macedonia had been warned that any violence could harm its chances of integration with the West.
The election campaign had been overshadowed by violence between rival ethnic Albanian parties.
There have been shootings and grenade attacks on party offices.
Recent opinion polls favoured the VMRO-DPMNE, suggesting it would take between 26% and 31% of the vote.
Its nearest rival, Radmila Sekerinska's Social Democrats, was given between 10% and 11% by the same polls.