Ukrainians are widely expected to give a resounding endorsement to the overthrow of their most recently elected leader by voting on Sunday for presidential candidates promising closer ties with the West.
But the absence of more than 15 percent of the electorate, in Russian-annexed Crimea and two eastern regions where fighting with pro-Moscow rebels continued on Saturday, may mar any result - and leave the Kremlin questioning the victor's legitimacy.
European election monitors largely pulled out of Donetsk region for their own safety, citing a campaign of "terror" by pro-Russian separatists against Ukrainian electoral officials.
Polls make a billionaire confectionery magnate known as the "chocolate king" an overwhelming favourite in a vote expected to show a high turnout.
For many the biggest question is whether Petro Poroshenko, who has been a minister in the past, can take more than 50 percent to win outright in round one.
Poroshenko, 48, was a strong backer of the protests against Moscow-backed President Viktor Yanukovich last winter and has sought a quick victory by warning that new unrest might prevent a second voting round.
His closest, if distant, rival is former prime minister and wealthy former businesswoman Yulia Tymoshenko.
Officials say many polling stations in Ukraine's Russian-speaking regions will not open for fear of attack and only early on Sunday will they try to distribute ballot papers to those areas where voting may be possible.
Voting will start at 8am local time (0500 GMT) and end 12 hours later. Exit polls at 8pm (1700 GMT) will indicate the result and an official outcome is due before international monitors deliver their verdict on the process on Monday afternoon.
Meanwhile on Saturday, Ukrainian troops launched mortars at the village of Semyonovka, near the pro-Russian separatist stronghold of Slovyansk, on Saturday afternoon setting a barn on fire, according to residents.
There was no official confirmation of Kiev involvement.
Ukrainian police and military units are locked in an operation against pro-Russian gunmen to clear them out of eastern Ukraine, an area the separatists say should be part of Russia.