Opposition supporters have gathered at Ukraine's Supreme Court as it prepares to deliver its verdict on the validity of the disputed presidential poll.
The judges have been pondering the case all week
Pro-Western opposition candidate Viktor Yushchenko says the vote was rigged to allow PM Viktor Yanukovych to win.
Both sides now admit a new poll is necessary but differ on the timing and the format.
Mr Yushchenko wants a re-run of the second-round, but the government is pressing for a totally new poll.
Whatever conclusion Ukraine's Supreme Court draws over the fraud allegations, its verdict will not provide a final resolution, says the BBC's Steven Eke in the capital, Kiev.
WHERE THEY STAND ON THE PRESIDENTIAL ELECTION
President Kuchma: Wants a new election to be held
Viktor Yanukovych: Wants the contested second round of the elections to be annulled but, like Kuchma, wants new elections from scratch
Viktor Yushchenko: Wants a repeat of the second round run-off with Mr Yanukovych
If it decides that there were falsifications in the east of the country, but not significant enough to change the overall result, Ukraine will be plunged into a serious constitutional crisis, he says.
Mr Yanukovych may demand that the procedure for his inauguration as president be started, but parliament might try to prevent that, and the opposition has already said it will not accept such an outcome.
- In the alternative scenario, where the Supreme Court annuls the results in the disputed eastern constituencies, there would be a requirement to hold some sort of run-off election.
Mr Yushchenko wants the re-run of the second round only in order to capitalise on the momentum he has built up on the streets and in parliament, our correspondent says.
The opposing camp, led by outgoing President Leonid Kuchma, may put forward a new man in a new poll - the former chief of the national bank and Economy Minister, Serhiy Tyhypko, is being tipped as the most likely candidate.
But analysts say a new poll could take several months to organise.
The stand-off in the streets of Kiev continues
In the meantime, Mr Kuchma has said he will choose a replacement for Mr Yanukovych, who lost a confidence vote in parliament on Thursday, if lawmakers agree a plan to dilute the powers of the president.
On Friday, he told parliamentary factions discussing the proposals that the political conflict was pushing the country into an economic crisis.
"The longer we drag out a political solution, the greater the economic damage will be and the closer we will get to irreversible processes," Mr Kuchma said.
Mr Yushchenko's insistence on a re-run of only the second round of the election was backed by Aleksander Kwasniewski, President of Ukraine's western neighbour, Poland.
"Elections have not yet been completed. We need to repeat the second round between the same candidates and give Ukrainians a chance to make a free choice," Mr Kwasniewski said.
But President Putin was more sceptical, and backed calls for an entirely fresh poll to pre-empt further challenges and re-runs.
Mr Putin also expressed concern about a possible split between the west of Ukraine, which generally supports Mr Yushchenko, and the east, which tends to lean towards Russia.
Hundreds of thousands of opposition protesters have been on the streets of Kiev for nearly two weeks, while government supporters have massed in the eastern city of Donetsk.