Voting has ended in Armenia's presidential election, with a leading opposition candidate alleging widespread irregularities.
Mr Sarkisian faces a strong challenge to his presidential hopes
"Very dirty things are happening," said former President Levon Ter-Petrosian, seen as the main rival of Prime Minister Serge Sarkisian.
Mr Ter-Petrosian's supporters plan to hold a protest rally in the capital Yerevan on Wednesday.
Seven others were also in the race. The results are due early on Wednesday.
Mr Sarkisian is a close ally of the outgoing Armenian President, Robert Kocharian.
Although small, the former Soviet republic 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.
Candidates were split over whether to offer more concessions to resolve the conflict with Azerbaijan over Nagorno-Karabakh.
Armenia is a mountainous country of 3.2 million people with a recent history of armed conflict and economic devastation - and it still has major problems.
The election was initially expected to be a smooth handover of power from Mr Kocharian to Mr Sarkisian.
But it turned into a much more volatile contest, the BBC's Matthew Collin reports from Yerevan.
In his campaign advertising, Mr Sarkisian promised to bring the country new prosperity.
But the campaign was brought to life by Mr Ter-Petrosian, making a dramatic comeback as an opposition candidate.
He has accused the government of not doing enough to resolve the Nagorno-Karabakh conflict, although some Armenians see his calls for compromise as a betrayal.
He said the authorities had "completely blocked us from any access to television".
The Armenian authorities strongly deny that they are manipulating the election process.