Sunday's vote began at 0500 GMT, but more than two million illegible voters were expected to be unable to cast their ballots as almost half of the polling stations were blocked in the rebel-held regions.
The election is being seen as an attempt by Ukraine's rulers to stabilise the country following recent political upheavals including the overthrow of its Russia-backed president, which prompted Russia's annexation of Crimean peninsula and a pro-Russia uprising in the east, Al Jazeera reported.
Arseniy Yatsenyuk, Ukraine's interim prime minister, has issued an appeal for the country's 36 million voters to turn out in force to "defend Ukraine" in the most important election since independence in 1991.
"This will be the expression of the will of Ukrainians from the west, east, north and south," he said on Saturday.
Pre-election polls suggested that Petro Poroshenko, an experienced politician and the country's seventh richest man, had a chance of winning the elections in the first round.
In case of a second round, he would have to face Yulia Tymoshenko, former prime minister and a long-time rival, who was released from prison in February, following the overthrow of President Viktor Yanukovich.
Ukraine has mobilised more than 55,000 police and 20,000 volunteers to ensure security for the vote, being overseen by 1,200 international monitors.
The country fears a new wave of violence by pro-Russian separatists who started their uprising seven weeks ago and declared independence of the Luhansk and Donetsk regions, prompting clashes with security services and demonstrators for united Ukraine. At least 150 people died in the unrest.