Early results in the parliamentary poll in Ukraine suggest a dramatic setback for President Viktor Yushchenko.
The count is not expected to be completed until Tuesday
The pro-Russian party of former PM Viktor Yanukovych - the man who lost the 2004 presidential election - looks set to take the largest vote share.
However, the president's party is in coalition talks with his former ally Yulia Tymoshenko's second-placed party.
The Orange Revolution allies fell out last year but they seem likely to reunite, correspondents say.
If confirmed, the results would be a humiliating blow for Mr Yushchenko, whose popularity has plunged since he took office last year.
With 10% of the vote counted, election commission officials put Mr Yanukovych's party in the lead with 24.7%, closely followed by Ms Tymoshenko's party with 23.8%.
The president's party trails in third place with 17.11%.
Counting of ballots is not expected to be completed until Tuesday.
A victory for Mr Yanukovych - who was written off a year ago - would be a dramatic comeback.
He was declared the winner of the presidential election in November 2004, but allegations of widespread vote-rigging sent hundreds of thousands of Ukrainians out on to the streets to demand change.
In what became known as the Orange Revolution, the election result was later overturned and Mr Yushchenko went on to win a re-run.
'Calling the shots'
The fall in Mr Yushchenko's popularity follows a year of political infighting and sluggish economic growth.
The team which emerged from the Orange Revolution split into competing factions following a power struggle. Ms Tymoshenko was sacked as prime minister amid allegations of corruption.
Now it looks like the Orange team will join forces again, but this time with Ms Tymoshenko calling the shots, the BBC's Helen Fawkes reports from Kiev.
Ms Tymoshenko has said she wants to return to the post of prime minister.
"We said repeatedly that this election was about choosing the prime minister and now the people have spoken," she said.
Ms Tymoshenko has also vowed to cancel a controversial gas deal Ukraine signed with Russia.
Both of these proposals would be difficult for Mr Yushchenko to accept but he has been left with few options, our correspondent says.
Exit polls suggested the party of Mr Yanukovych's party had taken a third of the vote.
"Today's victory is a revelatory moment for both myself and the Party of Regions," Mr Yanukovych said.
"It has shown that despite everything, the people have managed to show their great support of our political force."
Mr Yanukovych said he would support ties with the European Union, as well as mending Ukraine's relationship with Moscow.
Election officials said the ballots were unlikely to be fully counted before Tuesday because of the 45 parties represented.