"This day ... is a day of first fair democratic elections in Ukraine," Viktor Yushchenko said at a polling station where he and his wife voted.
The president, who came to power pledging to build closer ties with the West, faces a decisive test in Sunday's vote, with the Russia-friendly party of his rival Viktor Yanukovych poised to win most of the votes in the poll.
As none of the 45 parties vying to get into the Upper Rada legislature is expected to get enough votes to secure half the seats in the chamber, the power in the legislature will lie with those able to form a majority coalition.
Voters stood in long lines in early morning sunshine at polling stations in Kiev, ready to make their choice on a ballot paper nearly a metre in length.
Yushchenko said he was sure his party would be successful.
"I am in a great mood, a mood that comes before victory," he said, flanked by his wife and five children. "A democratic election in Ukraine is already a victory."
Yushchenko came to power in Ukraine after launching mass protests in late 2004 when official results from a presidential election that he said was rigged handed victory to his pro-Russian rival, Viktor Yanukovych.
Amid the demonstrations, which became known as the "orange revolution" for the colour of Yushchenko's campaign, the supreme court threw out the ballot because of massive fraud and ordered a re-run vote that Yushchenko won.
Western election observers said they did not expect to find evidence of the fraud that marred the 2004 campaign.