Ukrainian voters go to the polls to choose between Yulia Tymoshenko, the prime minister, and Viktor Yanukovich, the opposition leader, in a close presidential runoff that could yet disappoint hopes of a swift return to stability.
Polling stations open at 8am (06:00 GMT) on Sunday and the first exit polls are expected shortly after the end of the vote at 8pm (18:00 GMT).
Many commentators predict a narrow victory by opposition leader Yanukovich, but Tymoshenko, the prime minister, is threatening to summon protesters in a replay of the 2004 "Orange Revolution" if she deems the second-round election unfair.
Most analysts say the final vote will be closer and expect both sides to resort to legal manoeuvring and demonstrations if defeat looms.
East or West
The personal antagonism between Yanukovich and Tymoshenko mirrors the gulf between Ukraine's pro-Russian east and nationalist west.
Both candidates say they want to integrate with Europe while improving ties with Moscow, though Tymoshenko is seen as more enthusiastic about the European Union.
Challenges to the result will only prolong instability in the country of 46 million people and will further hurt investor confidence in the crisis-hit economy.
They will also delay talks with the International Monetary Fund on a $16.4bn bail-out programme derailed by breached promises of fiscal restraint.
The euphoria of 2004, when protests overturned rigged results that gave Yanukovich victory, has evaporated after five years of falling living standards and paralysing political squabbles between president and prime minister.
Portrayed as Moscow's stooge in the "Orange Revolution", the 59-year-old Yanukovich is eyeing a comeback after a campaign dominated by smear and accusation.
He won 10 per cent more of the votes than Tymoshenko in the first round on January 17.
The 49-year-old former gas tycoon has accused Yanukovich of cheating after his Regions Party pushed through parliament amendments to voting rules after the first round.