Incumbent Serzh Sarkisian was set to win Armenia's presidential election Monday with 61.4 percent of the vote, according to official partial results, with his main rival contesting the result.
The Central Election Commission said early tallies from over 50 percent of the voting precincts showed former foreign minister Raffi Hovannisian in second place with 33.93 percent.
Hovannisian swiftly insisted he was the real winner and called on Sarkisian to concede defeat.
Eduard Sharmazanov, spokesman of Sarkisian's ruling Republican Party, said exit polls showed the president "was the only favourite" and called the vote "the best in the history of independent Armenia", rejecting allegations of fraud.
But, referring to himself, Hovannisian said: "Our people deserve a de jure elected president."
Sarkisian "must become Armenia's first president who recognises the people's victory," he insisted.
Hovannisian's camp has alleged a range of sometimes bizarre electoral violations, including the use of "vanishing ink" to allow multiple voting and "caravans" of taxis and buses to take pro-government voters to the polls.
Police have dismissed the allegations as an "obvious fiction".
Voter turnout was 60 percent in the polls seen as a crucial democratic test for the former Soviet state.
A Gallup exit poll found Sarkisian, president since 2008, set for re-election to another five-year term with 58 percent of the vote with 32 percent of votes going to Hovannisian.
The five other candidates were all said to be scoring in single digits.
Former prime minister Hrant Bagratian was on course for three percent, as was the Soviet-era dissident Paruyr Hayrikyan, according to the exit poll.
The outcome had become predictable back in December when the highly popular leader of the Prosperous Armenia party -- super-rich former arm-wrestling champion Gagik Tsarukian -- said he was out of the race and Armenia's first post-Soviet president Levon Ter-Petrosian said he was too old for the country's top job.
The vote was clouded both by the lack of strong opposition to the incumbent, and a mysterious assassination attempt against Hayrikyan last month.
The authorities were hoping for a peaceful process that would improve the country's chances of European integration.
The vote that brought Sarkisian to power in 2008 ended in clashes in which 10 people died.
Hovannisian said the election marked "the most crucial day in our country's modern history" but denounced irregularities in voters' lists and voting procedures.
"These were shameful elections with a huge number of violations. The results of the exit poll do not show reality but what the authorities wanted," Hovannisian's spokesman Hovsep Khurshudian told AFP.
He vowed that Hovannisian's supporters would stage a protest on Tuesday evening.
International observers from the Organisation for Security and Cooperation in Europe have monitored voting and are set to give their verdict on Tuesday.
Sarkisian, 59, is a veteran of the 1990s war with Azerbaijan over the disputed region of Nagorny Karabakh and derives much of his popularity from a tough can-do militaristic image.
Hovannisian, 54, was born in the United States and practised law in Los Angeles before moving to Armenia following its devastating December 1988 earthquake.
All the candidates have made populist promises to fight poverty and unemployment.
The World Bank estimates that 36 percent of Armenians live below the poverty line, while economic hardship and unemployment have driven nearly a million Armenians out of the country over the past two decades.
But campaigning has also focused on Armenia's long-running disputes with neighbours Turkey and Azerbaijan.
No final peace deal has been reached with Azerbaijan over the Armenian-controlled Azerbaijani region of Nagorny Karabakh as the risk of a new conflict remains palpable.