Viktor Yushchenko secured 54% of the vote on Sunday, compared to the 43% of his pro-Russia rival - Prime Minister Viktor Yanukovich, according to one research institute.
The exit poll statistics were provided by the independent Razumkov Ukrainian Centre for Economic and Political Studies and the Kiev International Institute for Sociology (KMIS).
But a second exit poll carried out by the government-linked polling agencies Socis and Social Monitoring also showed a Yushchenko victory, though a bit narrower with 49.4% compared to Yanukovich's 45.9%.
Official results were not expected before Monday and Yanukovich's campaign immediately dismissed the exit polls as "inaccurate and unscientific."
But speaking before an excited crowd of thousands of opposition supporters massed in Kiev's Independence Square, Yushchenko's campaign manager declared victory.
"The results of the exit polls show an unchallengeable lead by Yushchenko," said Olexander Zinchenko to wild cheers.
Thirteen years after gaining independence from the Soviet Union, Sunday's runoff election in Ukraine is a landmark event that will determine whether this nation of nearly 50 million turns toward Europe or remains under Moscow's shadow.
Some 37 million people were eligible to cast ballots in the runoff election, which was being observed by nearly 5000 international monitors.