Incumbent Filip Vujanovic declared victory in Sunday presidential polls in Montenegro but his sole challenger disputed his claim, describing it as a "coup d'etat" in a move that could plunge the tiny EU candidate Adriatic country into a political crisis.
Vujanovic said he had won 51.3 percent of the votes, with his rival Miodrag Lekic trailing behind with 48.7 percent.
But Lekic said that, with 97 percent of the vote counted, "citizens have brought me victory" at the polls.
Montenegro's state electoral commission has remained mum on the outcome of the polls for a five-year mandate for president, the largely ceremonial post.
According to the law, the commission has 24 hours after the polls close to pronounce official results of the vote.
No major irregularities were reported during the election day, with the turnout estimated at around 60 percent of some 511,000 registered voters, the commission earlier said.
Lekic, who has managed to get Montenegro's main opposition groups to overcome their bickering and back his candidacy, said he had "indications of fraud" and called on Vujanovic to "be serious and responsible."
"By declaring victory, he has committed a coup d'etat," Lekic said in a brief address to his supporters.
Although supporters of both candidates were noisily celebrating alleged victories, no incidents were reported in the capital Podgorica.
The election of the new president is the country's second since it proclaimed independence from Serbia, its decades-long partner, in 2006.
But it is seen as a test for the ruling coalition that has been in power, under the stewardship of Milo Djukanovic, a close ally of Vujanovic, since the break-up of Yugoslavia in the 1990s.
Victory of opposition candidate Lekic could bring instability to Montenegro, as Djukanovic holds the powerful post of prime minister, while his Democratic Party of Socialists' coalition has majority in the parliament.
If elected, Vujanovic, a 58-year-old lawyer, has promised to focus on strengthening Montenegro as a "democratic developed country" in order to boost its efforts to join the European Union and NATO.
"We will intensify European and Euro-Atlantic integration as they are a necessity for Montenegro," Vujanovic said after casting his ballot in the capital Podgorica.
One pro-Vujanovic voter, 40-year-old businessman Dejan Nuculovic, said he backed the incumbent president because he saw him "as a part of the policy that leads the country to the EU".
Braving an unseasonably cold and windy day, another voter, Vukasin Bacovic came early to cast his ballot for Lekic, who, he hoped, would "end the corruption that has been ruining Montenegro for years".
"Lekic is a great adversary of corruption. If, God willing, he becomes president, things will improve," the pensioner said.
Brussels opened EU accession talks with Montenegro in June, but the European Commission noted that Podgorica should do more to uphold the rule of law and crack down on organised crime and corruption.
After Croatia, Montenegro is next in line among the ex-Yugoslav republics to join the 27-nation bloc.
Lekic, 65, has made the fight against corruption and organised crime his priority.
"We live in a mire of hypocrisy and corruption, in a system that humiliates people. Montenegro must get its house in order," Lekic said during the campaign.
The opposition has constantly accused Djukanovic and his allies of corruption and political and economic monopoly.
Djukanovic, a veteran politician and an architect of Montenegro's independence, has himself been accused of corruption but has denounced the claims as "lies".
In 2006 he was named a suspect in an Italian probe into cigarette smuggling and people trafficking during the Balkan wars in the 1990s.
But in May 2009 an Italian court dropped the charges against him.
Experts say that corruption is deeply rooted in the country of 632,000 people struggling with an unemployment rate of 20 percent, and where the average monthly salary is about 480 euros ($615).
Montenegro's relatively undiversified economy relies heavily on foreign investment, which drove an economic boom between 2006 and 2008. Since then, the economy has slid, and public debt has reached 51 percent of gross domestic product.
Both Vujanovic and Lekic have avoided making any promises of leading a fast economic recovery.