Armenian Prime Minister Serge Sarkisian has won fiercely-contested presidential polls in the former Soviet republic, official results show.
Mr Sarkisian's main rival insists he is the winner
Mr Sarkisian won 52.9% of the vote, with his nearest rival, ex-President Levon Ter-Petrosian, trailing on 21.5%.
But Mr Ter-Petrosian said the vote had been rigged, and his backers staged a big rally in the capital Yerevan.
The Organisation for Security and Co-operation in Europe said the poll broadly met democratic standards.
"Yesterday's [Tuesday's] presidential election in Armenia was conducted mostly in line with the country's international commitments," the OSCE's observer mission said in a statement.
But it added that "further improvements are necessary to address remaining challenges".
Poll monitors from a Russian-led group of former Soviet republics said earlier the elections were "free and open", Russia's Itar-Tass news agency reported.
The election results suggest that Mr Sarkisian wins outright, avoiding a run-off.
Turnout was nearly 70% of 2.3m eligible voters, Armenia's election commission said. Seven other candidates were also in the race.
Mr Sarkisian is a close ally of outgoing President Robert Kocharian, and the election was initially expected to be a smooth handover of power to Mr Sarkisian.
Ter-Petrosian supporters say the poll was rigged
But Mr Ter-Petrosian - Armenia's president between 1991 and 1998 - surprised experts with a lively and outspoken campaign, and now insists he is the real winner.
"Very dirty things are happening," Mr Ter-Petrosian said after voting, alleging that the election had been rigged.
The Armenian authorities have strongly denied manipulating the election process.
On Wednesday, more than 15,000 supporters of Mr Ter-Petrosian staged the protest rally near Armenia's election commission building in central Yerevan.
Riot police have been deployed to prevent any disturbances.
Mr Ter-Petrosian's spokesman, Arman Musinian, told the BBC that the opposition would do all it could to prevent anger turning to violence.
"If there is unrest, it is the authorities that is responsible. Our side has no intention to initiate any unrest or clash, or anything else that is contrary to the law," he said.
Although small, Armenia is regarded as strategically important - lying between the energy-rich Caspian Sea and the gas and oil markets of southern Europe.
Correspondents say the main issues for Armenians are tackling unemployment and resolving long-running disputes with neighbouring Turkey and Azerbaijan, both of which have closed their borders with Armenia.