Opposition supporters allege widespread falsification of votes
European monitors have described Kyrgyzstan's presidential election as a "disappointment", saying it failed to meet key international standards.
The Organization of Security and Cooperation in Europe (OSCE) said Thursday's poll was "marred by many problems and irregularities."
Partial results show incumbent Kurmanbek Bakiyev with an overwhelming lead of 85% of the vote.
The main opposition candidate has denounced the vote as fraudulent.
Almazbek Atambayev has already claimed widespread ballot-stuffing and intimidation of monitors.
But the central election commission has ruled the vote valid.
The OSCE appeared to echo Mr Atambayev's claims with its assessment of the election.
"Sadly, this election did not show the progress we were hoping for and it again fell short of key standards Kyrgyzstan has committed to as a participating state of the OSCE," it said in a statement on Friday.
"Election day was marred by many problems and irregularities, including ballot box stuffing, inaccuracies in the voter lists and multiple voting," the statement went on.
"The process further deteriorated during the vote count and the tabulation of results, with observers evaluating this part of the process negatively in more than half of observations."
The OSCE also said that the "misuse of administrative resources and bias in the media coverage of the campaign" gave Mr Bakiyev an "unfair advantage" over his rivals.
Mr Bakiyev appeared to be on course for a landslide victory on Friday, with the central election commission saying he had won 86.2% of the vote with two-thirds of ballots counted.
Late on Thursday, opposition supporters of Mr Atambayev gathered outside his campaign headquarters for a concert and to express their discontent with the election process.
Many said they witnessed widespread violations such as groups of so-called carousel voters moving from one polling station to another.
The crowd dispersed when leaders of the opposition reassured them that further actions would be taken in the coming days.
Mr Bakiyev - who came to power four years ago off the back of a popular uprising which became known as the Tulip Revolution - had pledged to conduct free and fair elections.
But his political opponents claim their supporters have been threatened.
Mr Atambayev said he had to send his wife and daughter away from the country after being intimidated.
During the vote, clashes between opposition members and government forces were reported in the northern town of Balykchi.