Armenian Prime Minister Serzh Sarksyan won a presidential election, results showed on Wednesday, but opposition parties prepared protests in the capital after complaining that the contest was rigged.

Sarksyan, who has vowed to continue the policies of incumbent Robert Kocharyan, had an unassailable lead with 782,233 votes - or 56 percent - of the 1,408,712 ballots counted, a spokeswoman for the Central Election Committee said.

The result looked set to give Sarksyan outright victory and let him avoid a runoff with his closest rival, opposition leader and former President Levon Ter-Petrosyan, who had 310,792, or 22 percent, of votes counted, as calculated by Reuters.

The results were from 85 percent of the ballots counted. The commission's spokeswoman did not give percentages for candidates and said the body would wait until all ballots had been counted before doing so.

High in the Caucasus mountains, land-locked Armenia lies between Turkey and Azerbaijan in an important transit region for oil exports from the Caspian Sea to world markets.

It relies heavily on an alliance with Moscow and, despite a recent spurt in economic growth, remains poor.

Analysts warn a still-unresolved conflict with neighbouring Azerbaijan over the separatist territory of Nagorno-Karabakh could flare up again into violence.

Stalled efforts to reach a peace deal are likely to be the biggest challenge facing the new president.

Sarksyan's campaign team said late on Tuesday it was waiting for definitive official results before making a statement and that the priority was for a free and fair election.

But an opposition aide said Ter-Petrosyan, Armenia's first president after it won independence from the Soviet Union, was the real winner, and complained of violations including ballot-stuffing, kidnapping and the beating supporters.

Ter-Petrosyan's camp announced a protest rally in the capital Yerevan on Wednesday.

Election observers from the Organisation for Security and Cooperation in Europe will give their verdict on the vote on Wednesday. Their assessment is likely to be a key factor in whether the opposition protests build momentum.

Previous elections in Armenia have been followed by days of opposition protests alleging ballot fraud. A new round of protests will be a test for stability in a country which, in the 1990s, was rocked by political convulsions.