KIEV – Ukrainian leader Petro Poroshenko begged for forgiveness and a second chance Thursday as polls showed him facing all-but-certain defeat three days ahead of a presidential vote.

In a dramatic televised address, Poroshenko pleaded with voters to support him over Volodymyr Zelensky, a comedian with no political experience who has surged ahead in the polls.

"I once again ask you to forgive me. What did not work out hurts the most," Poroshenko said.

Polls show Zelensky, a 41-year-old standup comic and television star, sailing to victory in Sunday's second-round vote.

The latest survey on Thursday showed him winning 73 percent of the vote against only 27 percent for Poroshenko.

His rise has stunned Ukraine's political elite, who have struggled to revive the economy, root out corruption and put an end to a war with Moscow-backed separatists in the country's east.

Poroshenko, 53, has cast himself as the only Ukrainian politician who can take on Russia's Vladimir Putin and settle the war, which has claimed 13,000 lives.

He said Thursday that because of the conflict with Russia it would be "very risky to experiment with the post of the president and commander-in-chief".

He also warned that Ukraine risked defaulting on its debt after a Kiev court on Thursday ruled that his government's 2016 decision to nationalise the country's biggest lender was illegal.

The bank, PrivatBank, was owned at the time by Igor Kolomoysky, a controversial tycoon who owns the channel that broadcasts several of Zelensky's television shows.

Poroshenko said a default would mean "tens of millions" of people would lose their money and suggested the ruling was linked with Zelensky's campaign.

He called an emergency meeting of his security council, while Kiev's international partners including the EU, Washington and the World Bank said in coordinated statements they were closely monitoring the situation.

Ukraine's government has spent nearly 160 billion hryvnyas (around $6 billion) on recapitalising the lender to cover the losses caused by the withdrawal of money from the bank and poorly-secured loans.

Anticipation for debate

"You see they just cannot wait," Poroshenko said. "They've already started implementing their plans."

Supporters credit Poroshenko, who took power in 2014 after a popular uprising ousted a Kremlin-backed leader, with implementing important economic reforms, rebuilding the army, securing an Orthodox Church independent of Russia and winning visa-free travel to Europe.

But Zelensky has seized on continued economic hardship and corruption, as well as his popularity as the star of sitcom "Servant of the People", in which he plays a school teacher who becomes a president.

Anticipation has been building ahead of a presidential debate scheduled for Friday in a 70,000-seat arena, the first and only one of the presidential campaign.

"Preparations for the debate are in full swing. You will see the results tomorrow," a Zelensky spokeswoman told AFP.

The comic suggested he would have an easy time in the debate given Poroshenko's record.

"I don't want to call anyone names, insult anyone or engage in mud-slinging," he told local media.

"After what has been done over the last five years I believe I can simply keep mum."

In Moscow, Kremlin spokesman Dmitry Peskov said he was hoping to watch what was likely to be a "curious spectacle".

Former prime minister Yulia Tymoshenko, who was knocked out in the first round of voting last month, said it was time to accept that Ukraine was facing a new political reality.

"Mathematics already tells us that Volodymyr Zelensky will win the second round," she said in a video address.

"This is objective reality. It's time to think of how to live with this."

Poroshenko has fought hard to recover lost ground in a bruising campaign after Zelensky leapfrogged the political establishment in the first round of voting on March 31.

Over the past few days the candidates traded insults on television and in social media. In a campaign that has become increasingly sensational, both men even underwent drug tests at Zelensky's insistence. – AFP