The monitoring mission of the Organization for Security and Co-operation in Europe said Sunday's vote failed to meet international standards and was marked by violence, intimidation and ballot box stuffing in predominantly ethnic Albanian areas.
"Organized efforts to violently disrupt the process early on Election Day made it impossible for voters in many places to freely express their will," it said.
Nikola Gruevski's center-right conservatives won 48.3 percent of the vote, far ahead of the Social Democrats' 23.4 and enough to give him a majority in the 120-seat parliament.
But one person was killed and eight wounded in gunbattles in ethnic Albanian areas. Along with allegations of fraud, the violence highlighted dangerous divisions within Macedonia's ethnic Albanian minority and threatened to undermine the Balkan nation's aspirations to join the European Union and NATO.
"What we witnessed here is damaging to this country's declared course to European and trans-Atlantic integration," said Mevlut Cavusoglu, head of a delegation from the Council of Europe's parliamentary assembly. "To be frank, I am very upset with what I witnessed in this country at these elections."
Voting was suspended at 22 polling stations 1 percent of the country's total in ethnic Albanian areas because of intimidation, violence or reports of fraud. Reruns will be held in those areas in two weeks.
"It is clear that the commitments to the Council of Europe standards and the OSCE standards in this particular election were not met," said Robert Barry, head of the observer mission.
Voting violations were registered in 30 polling stations, and 28 people were arrested, police spokesman Ivo Kotevski said.
Gruevski pledged to ensure a fair rerun, and vowed to prosecute those responsible, according to the MIA news agency.
"I will not form the new government as long as I am not certain that fair and democratic elections have been conducted in those 1 or 2 percent of the polling stations," he was quoted as saying.
"In a state that functions under the rule of law there is no room for violence. All those who intend to provoke such acts will be sanctioned, regardless of their political, ethnic and religious affiliation."
U.S. State Department spokesman Sean McCormack said the U.S. expects Macedonia to prosecute those involved.
"It's the responsibility (of) a government to provide an atmosphere where people are able to freely express themselves via the ballot box and feel assured that they wouldn't be subject to threat or intimidation," he said.
Germany called for the reruns to be trouble-free. Foreign Ministry spokesman Andreas Peschke said the election problems "undermine the trust of the people in the democratic processes."
EU foreign policy chief Javier Solana also condemned the violence and said that "key European commitments were not met."
He said the EU's overall assessment "will depend on the further handling and investigation of the violations identified."
Macedonia could face long-term consequences from Sunday's vote, said Biljana Vankovska, a political analyst and professor at Skopje university's Institute for Defense and Peace Studies.
"The international standing of the country has been undermined seriously," she said. "Yesterday was probably for many of us the worst possible outcome."
Ethnic Albanians make up about a quarter of Macedonia's 2.1 million people, and ethnic Albanian rebels fought a six-month insurgency in 2001. But divisions have grown between the minority's two main parties: the Democratic Union for Integration led by former rebel leader Ali Ahmeti and Menduh Thaci's Democratic Party of Albanians.
Tension escalated after the 2006 election, when Gruevski picked the DPA as a governing coalition partner even though it won fewer votes than Ahmeti's DUI.
"Time is long overdue for an intra-Albanian dialogue that could pave the way to reconciliation and enhance stability and democracy," Cavusoglu said.
The DUI won 11.1 percent of the vote Sunday, slightly ahead of the DPA's 10.2.
DUI called Monday for an investigation into DPA and Thaci for the shootings and other violations. Thaci also called for "people who fired bullets on civilians and policemen to be put behind bars."