The Regions Party, headed by Viktor Yanukovych who lost 2004's "orange revolution" presidential ballot to the pro-Western Victor Yushchenko, has gained 33 percent of votes counted so far, more than 10 points ahead of its nearest rival.
Orange revolution heroine Yulia Tymoshenko is apparently in second place with 23 percent of the vote, according to results of an exit poll done by the Razumkov Centre and the Kiev International Institute of Sociology.
Ukrainians cast their ballots for the new parliament on Sunday, the first test of the feuding "orange revolution" leadership and its pro-Western stance.
Mr Yushchenko's popularity has tumbled in the face of a sluggish economy and the acrimonious breakup of the feuding orange tea, and the pro-Russian camp is poised to make a strong comeback.
Earlier, the president described the vote as a choice between "the past and the future," and hailed it as the "first fair, democratic elections in Ukraine".
He also called on the bickering forces that backed him during the orange revolution to reunite after the election.
"I am in a great mood, a mood that comes before victory," the president, sporting an orange tie, told reporters after voting in central Kiev near the spot from where he led the protests in late 2004 that catapulted him to power.
His rival Mr Yanukovych also expressed optimism for a win, after promising economic prosperity, stability and improved ties with Moscow during his campaign.
The last pre-election opinion polls released two weeks before the vote suggested Mr Yanukovych's Regions Party would get up to 30 percent of the vote and Mr Yushchenko's Our Ukraine up to 20 percent, while Yulia Tymoshenko, the president's estranged "orange" partner, would come in third with up to 17 percent.
The poll is being watched closely both in the West and in Russia, with Ukraine in a strategic position.
While the Kremlin sees Ukraine as its historic backyard, the West has encouraged Mr Yushchenko's drive to enter its ranks, safe from an increasingly confident and authoritarian Moscow. Ukraine is deeply split along the divide.
In addition to casting ballots for the legislature, under a purely proportional system using party lists, voters also voted for regional and local assemblies and municipal heads.
Major queues formed outside polling stations as voters struggled to handle up to four different ballots, some of them up to a metre long, with some waiting several hours to vote.