Kiev (AFP) - A comedian tipped to take over Ukraine’s presidency vowed on Friday to dismantle the old political system as he went head-to-head with his incumbent rival in an extraordinary stadium debate before this weekend’s vote.

Polls show Volodymyr Zelensky, a 41-year-old standup comic with no political experience, handily defeating President Petro Poroshenko in a second-round of voting on Sunday.

The hour-long debate at Kiev’s Olympic Stadium capped a weeks-long sensational campaign in a war-torn country seen as a buffer between the European Union and Russia.

“I am not a politician,” said Zelensky, opening the first and only policy debate of the presidential race.

“I am just a simple person who has come to break down this system,” he added.

“I am the result of your mistakes and promises,” he told the 53-year-old president as the spectators clapped and jeered.

The television star’s bid was initially dismissed as a joke but he leapfrogged establishment candidates amid frustration over corruption, economic trouble and a conflict with Moscow-backed insurgents in the country’s east.

Police said more than 20,000 spectators were in attendance.

- Theatrics and showmanship -

At times Poroshenko – who wore a suit but no tie – seemed more confident but the consummate showman Zelensky dominated the head-to-head, often putting the Ukrainian leader on the defensive.

Dressed in a smart suit, Zelensky delivered a strong performance in which he demonstrated his trademark showmanship and a bit of theatrics, flashing victory signs and peppering his speech with catchy slogans.

At one point he went on one knee in honour of those who died in the war with insurgents, prompting Poroshenko to do the same.

The rivals opened the debate on a positive note by shaking hands but quickly launched mutual attacks.

Behind them stood their wives, teams and backers including fatigue-clad servicemen who supported Poroshenko.

The Ukrainian leader attacked the inexperience of the untested Zelensky and said he tried to be many things to many people.

“We only have a beautiful, bright sweet wrapper in which everyone can find what he is looking for,” said an emotional Poroshenko.

He slammed Zelensky for avoiding active-duty military service and added he would not be able to stand up to Russian President Vladimir Putin.

“An actor without any experience whatsoever cannot lead a war against an aggressor.”

Poroshenko also zeroed in on Zelensky’s ties to controversial tycoon Igor Kolomoysky whose TV channel broadcasts the comedian’s shows.

Zelensky said the self-exiled tycoon would go to jail if he violated any laws.

- ‘Poroshenko lost’ -

Many observers said the comic won the debate.

“Poroshenko lost,” author and producer Illarion Pavliuk said on Facebook. “Whether you like it or not.”

Russian foreign ministry spokeswoman Maria Zakharova dismissed the event a “circus”.

The debate brought to a close a race that has at times descended into farce as the candidates exchanged insults and underwent drug tests.

A survey released Thursday showed Zelensky winning 73 percent of the vote against 27 percent for Poroshenko.

Supporters credit Poroshenko – who took power after a popular uprising in 2014 – with rebuilding the army, securing an Orthodox Church independent of Russia and winning visa-free travel to Europe.

However, he won just over half of Zelensky’s vote share in the first round of the presidential election last month.

The stakes are high for the country of 45 million people which dreams of joining the European Union one day.

Ukraine is mired in a smouldering conflict with separatists in the industrial east that has claimed 13,000 lives.

The West is closely watching the race amid concern that a new government might undo years of reforms.

The main risks associated with a Zelensky presidency are “policy incoherence stemming from inexperience” and “undue influence from oligarchs or Russians,” a former Western diplomat told AFP.