He also declared that Zelensky would be a weak president, to the benefit of Russian Federation, which annexed Crimea in 2014 and has been bankrolling separatists in Ukraine's east.
Ukraine's presidential rivals have traded insults in front of tens of thousands of people during their first and only debate.
The first round took place on March 31 and saw Zelensky receiving 30.24% of the votes, nearly twice as much as Poroshenko, who got 15.95%. "The main thing is not to lose the country", Poroshenko said. "I'm just an ordinary person who has come to break the system".
"I am not your opponent, I am your sentence", Zelenskiy told him. "How did it turn out that Ukraine is the poorest country with the richest president?"
Standing in front of a Ukrainian flag surrounded by army veterans, Poroshenko tried to portray Zelenskiy as an inept draft-dodger who would be unable to defend the country against Russian President Vladimir Putin. "Putin is dreaming of chaos".
Poroshenko also questioned Zelensky's ability to serve as commander-in-chief and called him a draft dodger. "Never again. The atmosphere is just super", said Anya, a 25-year-old Zelenskiy supporter from the Poltava region.
The two took the tests, but afterwards struggled to agree a date.
Zelenskiy has put Poroshenko on the backfoot by running an unorthodox campaign that eschewed traditional rallies and relied on quirky social media videos and comedy gigs. In his rare interviews he has voiced support for close ties with the European Union and North Atlantic Treaty Organisation but also talked about making more active efforts to re-integrate the rebel east back in Ukraine's fold.
Zelensky claimed 30.24 percent in the first round of the election on March 31, beating Poroshenko with an nearly twofold advantage (15.95 percent).
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