The government claimed its candidate, Prime Minister Viktor Yanukovych, won the first round. But opposition leader Viktor Yushchenko accused the government of cheating and claimed victory himself.
With 91 percent of districts counted, the Central Election Commission credited Yanukovich with 40.46 percent of the vote to almost 39 percent for Yushchenko.
But Yushchenko accused the government of distorting the results and said that the opposition's "parallel" calculations on the basis of exit polls from around a third of constituencies had given him an outright victory with 54 percent of the vote compared to 27 percent for Yanukovich.
"Firstly, dear friends, something that Ukrainian democracy has strived to achieve for such a long time has come to pass -- the triumph of Ukraine's democratic forces," Yushchenko said.
Yushchenko accused Yanukovych's camp of manipulating the vote in eastern areas, such as the prime minister's home region of Donetsk. He said that the government is readying to falsify the results in Donetsk, which alone constitutes 10 percent of voters in Ukraine. "The government has left this territory as a means to regulate the indicators for the election," he said.
Another exit poll gave Yushchenko 45.2 percent to 36.8 percent for the prime minister while a parallel count by the Yanukovich camp put the prime minister in the lead with 41.2 percent to 38.7 for the challenger.
Experts said some of the difference in the tallies could be explained by the fact that Yushchenko, Yanukovich, and the election commission received figures from different areas. Yushchenko's main area of support is in western Ukraine.
Some international election monitors also said there were reports of election fraud. The leader of a Canadian monitoring mission, John Mrzav, said: "We have had reports of violence in cities in eastern Ukraine where thugs have threatened the heads of polling stations unless they produce an 80 percent vote for Yanukovych. We are taking these reports very seriously and investigating them."
But Hanne Severinsen of the Council of Europe, a pan-European human rights body, praised the official voter turnout of some 75 percent and said the likelihood of a runoff means "democratic elections can really take place."
Yanukovych, meanwhile, was resting at home today and left it to his campaign chief, Serhiy Tyhypko, to comment on his lead in the polls. Tyhypko insisted the vote had been fair. "We managed to retain peace and calm in the country," he said. "With my hand on my heart I must say that I was very anxious about that. I always believed in the wisdom of the Ukrainian people and for this reason Ukraine will be a strong, unified, and blossoming country."
The election is seen as a choice for Ukraine between the West or Russia. Yushchenko advocates EU and NATO membership while Yanukovych wants Ukraine to draw closer to Russia and become part of a Moscow-led common market that also includes Belarus and Kazakhstan.
The final outcome in a runoff could depend on which man wins the most support from the field of 24 other candidates.
Socialist Oleksandr Moroz, third with more than 5 percent, has generally sympathized with challenger Yushchenko. Fourth-place Communist Petro Symonenko has given no indication whom he might back.
[For full coverage of Ukraine's presidential elections, see RFE/RL's Ukraine web page.]