Early results from Ukraine suggest the Prime Minister Viktor Yanukovych is leading in the presidential poll by a narrow margin.
The country is sharply divided
With more than 80% of the vote counted, Mr Yanukovych is ahead of opposition leader Viktor Yushchenko by 42% to 37%.
This suggests neither candidate will reach the 50% required for outright victory and they will go to a run-off.
However, international observers have reported significant irregularities acknowledged by officials in Kiev.
Mr Yanukovych, backed by outgoing President Leonid Kuchma, and the pro-Western Mr Yushchenko have fought a bitter election campaign marked by claims of dirty tricks and media bias.
Ukraine's 37 million voters were asked to choose between the 20 candidates contesting the election.
Mr Yanukovych's initially commanding lead was eroded as more results came in through the night.
Mr Yushchenko insisted exit polls and estimates put him in the lead, and told supporters and reporters in Kiev: "Democracy has won."
"The Ukrainian people showed today that the authorities can be defeated," he reportedly said.
European observer groups complain that across Ukraine, names have been missing from the electoral register. It has been reported that in one city, 5% of potential voters were affected.
They also reported incidents of intimidation and pressure on voters, said the European Network of Election Monitoring Organisations.
In an interview with Ukrainian TV, Central Electoral Commission head Sergei Kivalov acknowledged "the biggest problem has been the electoral register, everybody is talking about it".
He said on Ukrainian state-owned television UT1 that citizens had lodged complaints, and promised to "analyse each situation" after the vote. He said those responsible for the registration problems would be called to account.
Long queues were reported at some of the more than 30,000 polling stations nationwide.
Groups allied to the opposition have previously accused the government of running biased media coverage and alleged the government might try to rig the vote.
Yanukovych, right, is backed by both Kuchma and Putin
Mr Yushchenko alleged in September he had been deliberately poisoned.
Casting his vote, Mr Yanukovych said the outgoing government had a good record in improving livelihoods.
"This means above all social benefits and salary levels. I see no reason for Ukrainians to be worried," he said.
Mr Yushchenko, who advocates closer ties with the West, said: "I want to live in a proper country where people respect honest leaders and do not fear them... where there is rule of law and an honest legal system."
The elections are being closely watched by both Russia, which backs Mr Yanukovych, and the US, which has complained about a "deeply disappointing" campaign.