A noisy, boisterous standoff in a cavernous stadium. A police cordon separating the two rival fan sections, rock music pumping and insults flying. It had the feeling of a boxing grudge match, but instead it was the final act of Ukraine’s chaotic, unpredictable election campaign.

Polls suggest that despite a complete lack of political experience, the 41-year-old comedian and actor Volodymyr Zelenskiy is due to score an overwhelming victory over the incumbent, Petro Poroshenko, in Sunday’s vote.

Friday’s debate was seen as Poroshenko’s last chance to turn the tide and prove to the country that even if they were disappointed in his first term, sticking with him was a safer option than putting the country into the hands of a political novice.

The debate had been the main subject of conversation in Ukraine all week, and was broadcast live on several television channels. Police said 22,000 people came to Kyiv’s Olympic Stadium to listen.

Facebook Twitter Pinterest Ukraine’s presidential candidates debate in Kiev. Photograph: Vasily Fedosenko/Reuters

In the end, it was more of a shouting match than a debate. Poroshenko accused Zelenskiy of being a draft dodger, a political neophyte and dangerously pro-Russia. Zelenskiy shot back by reading a series of questions apparently sent in by ordinary Ukrainians, including “How do you sleep at night?” and “Why is Ukraine so poor when it has the richest president in its history?”

Poroshenko won the presidency in 2014 shortly after the Maidan revolution swept former president Viktor Yanukovych from power. As the army fought a war with Russia-backed forces in the east of the country that has now cost over 13,000 lives, the economy tanked and corruption flourished.

Many voters are disappointed by the lack of progress and are ready to vote for Zelenskiy despite his lack of a programme, in the hope he might shake up the system.

Play Video 3:02 Could this comedian be Ukraine's next president? – video explainer

Zelenskiy has no political experience except playing a fictional president on screen, but despite giving little away about concrete policy positions, he won the first round three weeks ago with 30% of the vote, compared with 16% for Poroshenko. Since then a series of polls have suggested he is on course for an overwhelming victory in the run-off.

“I’m not a politician. I’m a simple human who came to break the system,” he bellowed into the microphone, his voice bouncing the stadium to rival choruses of cheers and boos. “I am the result of your mistakes,” he told Poroshenko.

Zelenskiy has run an unorthodox campaign consisting of stand-up comedy gigs, Instagram videos and quips, and had eschewed direct debate until the stadium event.

Facebook Twitter Pinterest ‘Putin is dreaming of a weak and ill-prepared novice,’ Poroshenko tells the crowd. Photograph: Sergei Grits/AP

“He hides from journalists, from his opponents and from the Ukrainian people. Finally we can find out what exactly he plans to do,” said Poroshenko, his voice audibly emotional.

It was Zelenskiy’s idea to hold the stadium debate in the first place, laying down a video challenge to Poroshenko he expected the president to turn down, saying he would only take part in a debate if it took part in the Olympic Stadium and both candidates took a drugs test. Realising he had little to lose, Poroshenko agreed, inviting the cameras to film him giving blood, urine and hair samples earlier in the month.

After a long wrangle over the format, the decision was taken to have two separate stages at each end of the football pitch. Poroshenko subverted the format before the debate had even started, strutting the length of the pitch down the touchline and emerging on stage with Zelenskiy.

Zelenskiy’s half of the stadium was sparsely filled, reinforcing the feeling that his stellar polling figures are less a reflection of fanatical support for his candidacy, but instead represent a general dissatisfaction with Poroshenko and the political elite.

“By no means do we think that our opponent is a Kremlin agent, but Putin is dreaming of a weak and ill-prepared novice,” Poroshenko said, reinforcing his campaign message that Zelenskiy is a threat to national security. In response, Zelenskiy criticised Poroshenko for conducting business in Russia.

There was little discussion of concrete policies, and it did not seem that Poroshenko landed the kind of knock-out blow he needed to turn around the polls. After the debate finished, he walked down the touchline again waving goodbye to his supporters, and it may well be that he was waving goodbye to the presidency as well.

Barring a dramatic last-minute turnaround, Ukraine is heading for an uncertain new era under a mysterious new president.