Mr Altambayev said he planned to lead a protest in Bishkek
The main opposition candidate in Kyrgyzstan's presidential election, Almazbek Atambayev, has pulled out on polling day, claiming widespread fraud.
Mr Atambayev said the vote, in which President Kurmanbek Bakiyev is running for a second term, was illegitimate and a new election should be held.
The US and Russia both have military bases in the strategically important Central Asian nation.
Mr Bakiyev is widely expected to hold on to the presidency.
Mr Atambayev told a news conference in the capital Bishkek: "Due to mass voter fraud we demand that this election be stopped and a new election held instead."
He had earlier said he planned to lead a march to the election commission later in the evening, when preliminary results are to be announced.
"We will not be defeated," Mr Atambayev said after casting his ballot.
"People will march in an organised way... We will wait until this evening and then people will decide what to do for themselves."
An opposition rally of about 500 people in front of the mayor's office in the northern town of Balykchi was broken up by riot police, says the BBC's Rayhan Demytrie in Bishkek. No injuries were reported.
A spokesman for Mr Atambayev said police fired shots in the air and used batons to disperse the crowd, Reuters news agency reported.
The head of the election commission said the election was valid and that with several hours still to go before the polls closed, voter turnout was 61.83%.
An election monitor in Kyrgyzstan with the Organisation for Security and Co-operation in Europe (OSCE), told the BBC that there were concerns during the campaign that the media was strongly biased in favour of "one candidate".
The OSCE monitors release their report on the conduct of the election on Friday.
With Mr Atambayev out of the running, there are now five candidates, including Mr Bakiyev.
His opponents have accused him of stifling dissent and tightening his grip on power. There has been a series of attacks on independent journalists.
Kurmanbek Bakiyev - the incumbent leader
Almazbek Atambayev - former prime minister and the main opposition candidate
Temir Sariyev - who broke off from the coalition of opposition parties to run for president
Zhenishbek Nazaraliyev - a celebrity doctor who, if elected, promises to legalise opium cultivation
Toktaim Umetalieva - a female Krygyz activist
Nurlan Motuyev - an entrepreneur allegedly linked to a coal-mining scandal
Mr Bakiyev issued a warning to potential demonstrators, saying: "We will suppress, within the limits of the law, any attempts to organise disorder."
Security is tight across Kyrgyzstan, with 5,000 officers deployed around the country and extra measures in the potentially restive areas of the Ferghana valley.
The US and Nato use the Manas airbase in Kyrgyzstan to supply their troops in Afghanistan, and will be watching intently for any signs of political instability, analysts say.
Kyrgyzstan is the only state in Central Asia to have had a so-called "colour" revolution, when a previous president was removed in a popular uprising.
That happened in 2005, but four years on the country finds itself in a different political environment, according to the BBC's Rayhan Demytrie in Bishkek.
President Bakiyev - who took power after the Tulip revolution with 90% of the vote - has campaigned on a platform of stability.
Out of five million Kyrgyz citizens, more than half are eligible voters.