Thirty-nine candidates were listed on the ballot, but the race narrowed to three main contenders: Poroshenko, who is seeking a second five-year term; Tymoshenko, a former Prime Minister and political prisoner; and Zelenskiy, a popular comedian and political novice.
All three said they want to continue building closer ties to Western institutions such as the European Union. But the election shone a spotlight on popular disdain for the political elite and the potential for continuing instability in one of Europe's most geopolitically pivotal countries.
Polls had shown Zelenskiy, 41, to be the favourite going into election day. He has largely led opinion polling since he announced his candidacy on his TV variety show on New Year's Eve.
Balazs Jarabik, an expert at the Carnegie Endowment for International Peace, said the significance of the results so far was that Zelenskiy "held a commanding lead in every region, including western Ukraine.
"This means that Zelenskiy has a chance to turn the second round into a referendum on Poroshenko's rule — instead of as a vote against Putin, as Poroshenko wanted," he said.
Tymoshenko released her own exit poll showing her in second place.
"I'm convinced that after a real count of the results, that after the collection of all the official results, the mafia that Poroshenko leads will not be in power," she said after the polls closed.
Zelenskiy has burrowed into a deep vein of voter discontent over a sluggish economy, corruption and an unresolved war in the country's east with Russian forces and Kremlin-backed rebels. Five years after the revolution, Russia continues to occupy the Ukrainian peninsula of Crimea, and the United Nations estimates that the war in the east has taken around 13,000 lives.
"Today we have a new life for our country," Zelenskiy said after he arrived at his polling station in Kiev, surrounded by journalists. "A new life begins, one that is normal, without corruption and without bribes."
In a case of life imitating art — or in this case, Netflix — Zelenskiy's only claim to presidential experience is that he plays a commander-in-chief in a popular television series, Servant of the People. His character, Vasyl Petrovych Holoborodko, is a simple but upright schoolteacher, who is unexpectedly catapulted to the presidency and tackles the country's venal oligarch class.
Among Zelenskiy's supporters, the series' message resonates that honesty among politicians should trump all other considerations, and that he himself in real life is seen to be decent and corruption-free.
"I like that he built his business without any government connections," Vitaly Kyrnik, 37, a railroad worker, said at a performance of Zelenskiy's comedy troupe, Kvartal 95, in Kiev at the weekend.
"He's a new person, young, with new views," Kyrnich added.
Poroshenko, 53, the owner of a confectionary company and one of the country's richest men, resurrected his electoral prospects, campaigned on patriotic themes, such as the promotion of the Ukrainian language over Russian, support of the military and the creation of an independent Ukrainian Orthodox Church. One of his early campaign slogans was "Either Poroshenko or Putin" — a not-so-subtle hint that an election victory for anyone else would mean a victory for Russian President Vladimir Putin.
