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Ahead of Sunday's hotly contested presidential election, Ukraine is observing a so-called "day of silence" in which campaigning is forbidden.

The campaign hit a dramatic high on Friday night when the comic actor who is far ahead in public opinion polls debated with President Petro Poroshenko in the country's largest sports stadium.

Poroshenko denounced Volodymyr Zelenskiy, who has never held political office, saying he would be too weak and inexperienced to stand up to Russia's attempts to bring Ukraine back within its orbit. Zelenskiy in turn criticized Poroshenko for failing to rein in corruption during his five years in office.

In a melodramatic moment, both kneeled to ask forgiveness of those who have lost relatives in five years of fighting with Russia-backed separatists in eastern Ukraine.