First exit polls from Ukraine's bitterly fought presidential election point to a run-off win for pro-Western opposition leader Viktor Yushchenko.
Yanukovych and Yushchenko: Bitter rivals
He is predicted to get more than 50% of the vote, against just over 40% for the Prime Minister, Viktor Yanukovych.
The campaign team of Moscow-backed Mr Yanukovych has contested the figures, saying their candidate was in the lead.
Mr Yushchenko, who narrowly won the first voting round, has threatened mass protests if Sunday's result is rigged.
The capital, Kiev, is on high alert, with extra police and soldiers on the streets.
Ukraine's Central Election Commission is being guarded by at least four water cannon and two armoured personnel carriers.
Turnout was at 76% despite plummeting temperatures, election officials said.
Counting began immediately after the polls closed at 1800 GMT, with the first official results expected to be announced in the coming hours.
However, the BBC's Helen Fawkes in Kiev says it could take some time for the final figures to be known.
After the first round on 31 October, it was 10 days before the overall result was released.
Then, Mr Yushchenko garnered 39.87% of the vote, compared to Mr Yanukovych's 39.32%.
International observers and the opposition said the first round was a step backwards for democracy in the former Soviet republic of 48 million people, alleging widespread fraud and intimidation.
Sea of orange
Tens of thousands of opposition supporters have answered Mr Yushchenko's rallying call to gather in Kiev's main square.
Opposition supporters say their Kiev rally will go on overnight
A huge screen has been erected in the city's Independence Square to hold a parallel vote counting.
There were wild cheers among opposition supporters, clad in the orange campaign colours, after figures from the first two exit polls were released.
Mr Yushchenko's campaign chief, Okeksander Zinchenko, said the exit polls showed "a clear victory for our candidate", urging opposition backers to "defend this victory".
The first exit poll, conducted by several Ukrainian research organisations, gave Mr Yuchshenko 54% of the vote, against Mr Yanukovych's 43%.
The results were based on the answers of 77% of 30,000 voters questioned at nearly 500 polling stations around the country.
Another poll - by Ukraine's Social Monitoring Centre - put Mr Yushchenko ahead by 49.5% to 45.9%.
However, Mr Yanukovych's camp has described the figures as ridiculous.
Mr Yanukovych's campaign manager, Serhiy Tyhipko, said research by their own team showed the prime minister was in the lead by 3 to 5%.
Experts point out that some of the exit polls conducted after the first voting round had been inaccurate.
Both sides have complained of problems during Sunday's voting.
Imprisoned twice in his youth
Former governor of industrial Donetsk region
Raised pensions and public sector pay before election
Would make Russian second official language and allow dual citizenship
An economist and former central banker
Has an American wife
Promises to fight corruption, create five million jobs and pursue free market reforms
Would seek deeper relations with the Europe and the West
The authorities are investigating the killing of a policeman who was guarding ballot papers in a village in central Ukraine. The motive for the killing is not known.
Police said eight ballot boxes were set on fire in a western pro-Yushchenko part of Ukraine, AFP news agency reported.
Both the US and the European Union say future relations with Ukraine depend on Sunday's vote being conducted fairly.
During the campaign, Mr Yushchenko, prime minister between 1999 and 2001, claimed to have been the victim of intimidation and dirty tricks, including an alleged poisoning attempt.
His critics portray him as an American puppet who will do anything to gain power, including inciting civil unrest.
Moscow has made it clear it is fully behind Mr Yanukovych.
Shadow of violence
On the eve of the run-off ballot, Mr Kuchma - who is finishing his second consecutive term as president - warned in a televised address to the nation that the authorities would not allow the democratic process of the election "to turn into undemocratic violence."
He said: "There will be no revolution, but elections worthy of a European country in the 21st Century."
Mr Kuchma - who is backing Mr Yanukovych - implied that the opposition was threatening to take power by force.
"The authorities will never allow an aggressive minority to dictate political logic. We are already hearing calls for violence," he said.