Ukraine's President is running against -- Vladimir Putin

A woman casts her ballot at a polling station in Kiev during Sunday's election.

Kiev, Ukraine (CNN)The front-runners in Ukraine's presidential election were a chocolate magnate, an energy tycoon-turned-populist and a television comedian. That may sound like a parody headline, but Sunday's vote was serious business for a country at war.

Ukrainians went to the polls on Sunday after five years locked in a proxy war with Russia. In 2014, Russian troops annexed the Black Sea peninsula of Crimea, and Moscow fueled a separatist conflict in eastern Ukraine's Donbas region. The United Nations estimates that, as of February 15, the fighting has claimed almost 13,000 lives, with at least 3,321 civilian deaths and an estimated 9,500 combatants killed.
The incumbent, President Petro Poroshenko, campaigned on his ability to play tough with Russia. In a statement this week on Twitter, Poroshenko cast Russian President Vladimir Putin as his main opponent.
"When asked who is my ally, with whom I am ready to unite and coordinate my actions, I answer: my ally is the Ukrainian people," he tweeted Tuesday. "Who is my opponent? I am not ashamed to say it openly -- this opponent is Putin."
    But Sunday's vote ended without any candidate obtaining an absolute majority of votes, so a runoff will be held April 21.
    Candidate Volodymyr Zelensky, a television comedian and political newcomer, received 30.4% of the vote, according to exit polling posted by Ukrainian state news agency Ukrinform.
    Poroshenko, the incumbent, appears likely to move to the second round, with 17.8% of the vote in early exit polling.
    Another top candidate, former Prime Minister Yulia Tymoshenko, carried 14.2% of the vote, according to the poll. In remarks at a press conference, Tymoshenko conceded first place to Zelensky, but discounted exit polls showing Poroshenko in second place.
    Poroshenko faced an uphill battle to make it to that second round. Earlier this week, a poll by Rating Group Ukraine showed that the leading candidate, Zelensky, had the support of 27.7% of intended voters.
    Just under a year ago, the President's support among likely voters was in the single digits. Since then, Poroshenko has reduced the gap: In another recent survey, American pollster GQR found the current president catching up with Zelensky, with 20% of likely voters saying they would vote for him if the election were held the coming Sunday.