Comedian Volodymyr Zelensky swept to victory in Ukraine's presidential election today, an exit poll showed, as millions of voters weary of war and economic hardship rebuked the ruling elites and ushered in fresh uncertainty for their geopolitically pivotal nation.

Zelensky, a 41-year-old TV star with no political track record, won 73 per cent of the vote in the runoff election, according to national exit poll results broadcast by Ukrainian television.

President Petro Poroshenko, who was running for his second five-year term, received just 25 per cent, according to the poll.

"To all Ukrainians, no matter where you are, I promise that I will never let you down," Zelensky said after the results came in. "Though I'm still not president, I can say as a Ukrainian citizen to all the countries of the former Soviet Union: Look at us. Everything is possible."

The comedian had been heavily favoured going into the election, holding a decisive lead in the polls in recent weeks. His surge to the top of a crowded field of candidates in the presidential campaign's first round, and his apparent landslide win today, reflected the disdain with which many Ukrainians see their political establishment five years after their pro-Western revolution.

Zelensky's experience in politics amounts to playing the Ukrainian president on a popular sitcom. But amid a continuing war in eastern Ukraine, economic travails and popular revulsion over allegations of government corruption, Zelensky's anti-establishment, antiwar and reformist message captured the support of a wide cross-section of the country.

"I voted for Zelensky because everything he said is true," said Viktoriia Bengalska, a 45-year-old secretary in Kiev. "It's impossible to survive on this salary, prices have increased like crazy, and we were promised something totally different."

Zelensky's apparent victory is the latest in the global trend of political outsiders harnessing TV and social media to outmuscle the unpopular establishment.

It is likely to reverberate in Russia and elsewhere across the former Soviet Union, where few other countries can claim a democratic system that would allow a comedian to unseat the sitting president. And it prompts questions about Ukraine's strategy in its conflict with Russian President Vladimir Putin - with whom Zelensky has promised to negotiate while not detailing how.

"Zelensky doesn't have experience, and Putin is a very dangerous adversary," said Volodymyr Fesenko, a political analyst in Kiev. "There's a lot of risk here."

Ukraine, a country of more than 40 million people, is pivotal to Putin's effort to maintain a sphere of influence in Eastern Europe. Putin annexed Ukraine's Crimea Peninsula and backed separatists in eastern Ukraine after popular protests toppled a Moscow-friendly president in Kiev in 2014.

A successful democracy in Ukraine - which shares deep historic, linguistic and cultural links with Russia - could also energise opposition within Russia to Putin's autocratic rule.

In Russia, both pro-Kremlin and independent news media broadcast live feeds of Saturday's stadium debate between Poroshenko and Zelensky, a spectacle offering a striking contrast to the lack of a domestic challenge to Putin. "We want it like in Ukraine," said a headline in the Russian business newspaper Vedomosti.

Zelensky has said he will maintain Ukraine's pro-Western course, and he has pledged not to give away any territory in negotiations with Putin. But he has signalled greater flexibility than Poroshenko in potential negotiations over the war in eastern Ukraine, sparking optimism as well as concern that he could be outmatched by the Kremlin.

"We would not advise him to talk to Putin alone," a Western diplomat in Kiev said.

But many voters brushed those fears aside amid intense dissatisfaction with their current leadership. The war in eastern Ukraine has killed about 13,000 people, according to the United Nations. Sporadic, deadly shooting continues to occur, and Russian-backed rebels occupy a swath of territory around two major cities near the Russian border.

Poroshenko, whose confectionery business makes him one of the country's richest men, took office in 2014 in the wake of Ukraine's pro-Western revolution. He built his campaign around the theme of independence from neighboring Russia - strengthening the military, promoting the Ukrainian language over Russian, and forming a Ukrainian Orthodox Church separate from Moscow. His slogan: "Army! Language! Faith!"

In his last-ditch appeal before the runoff, Poroshenko told voters that handing the presidency to Zelensky would imperil the very existence of the country. Zelensky's slick, social media and TV-driven campaign masked the influence of the Kremlin and of unscrupulous billionaires, Poroshenko alleged.

"This is a bright candy wrapper," Poroshenko said in the debate, referring to Zelensky. "There are Russians inside, and fugitive oligarchs."

Those concerns echoed among Poroshenko's supporters, a sign that one of Zelensky's most difficult tasks will be to bring together a divided country. At a polling station set up at a Kiev public school, Inna Dzhurynska, 52, pointed at her traditional Ukrainian embroidered shirt when asked whom she was voting for.

"Who do you think I will vote for?" Dzhurynska said. "We'll lose Ukraine with Zelensky," she added, and broke into tears.

During his campaign, Zelensky largely eschewed traditional advertising and unscripted interactions with journalists. Instead, the entertainer relied on social media and his television shows to reach voters.

On his sitcom, Zelensky plays a simple, morally upright schoolteacher who is elected president after his rant of outrage over corruption is caught on camera and goes viral. He then takes on Ukraine's entrenched business and political elites, refusing to be bought.

The third season of Zelensky's show, "Servant of the People," aired this northern spring and includes scenes of a prosperous, corruption-free Ukraine in the aftermath of the Zelensky character's presidency.

"I'm not a politician," Zelensky said, channelling his character in his show. "I'm just a simple person who came to break the system."

To be sure, his real-life political rise isn't quite the Cinderella story told in his sitcom. The long-popular entertainer has benefited from his business partnership with Ukrainian billionaire Ihor Kolomoisky, who controls the television channel that airs Zelensky's shows and gave largely positive coverage to his candidacy. Both men deny that Kolomoisky is behind Zelensky's political ambitions.

Zelensky's most powerful advantage in the runoff may have been simply that he was not Poroshenko. Many voters blame the incumbent for the failure to end the war in eastern Ukraine and for allowing corruption to fester at the highest levels of government. Poroshenko countered that it's Russia's fault that the war lingers and that he did what was possible to reform governance in Ukraine.