In a brief stand-off at border control after arriving on a flight from Warsaw on Monday, Poroshenko accused border guards of taking away his passport. He later greeted a crowd of thousands of cheering and flag-waving supporters outside the airport.
Poroshenko's return sets up a showdown with President Zelenskiy's government in what critics say is an ill-judged distraction as Ukraine braces for a possible Russian military offensive and seeks the support of its Western allies.
Western diplomats called for political unity in Ukraine before the arrival of Poroshenko, who was president from 2014 until 2019.
Poroshenko, 56, is being investigated for alleged treason linked to the financing of Russian-backed separatist fighters through illegal coal sales in 2014-15. His party accused Zelenskiy of trying to silence political opposition.
Zelenskiy's administration says the prosecutors and judiciary are independent and accuses Poroshenko of thinking he is above the law. Zelenskiy trounced Poroshenko in an election in 2019. He ran on a ticket to tackle corruption and curb the influence of oligarchs in the former Soviet republic.
"We are not here to protect Poroshenko, but to unite and protect Ukraine," Poroshenko told the crowd, before heading to the Pechersk court in central Kyiv for a hearing, as supporters chanted outside the building.
During a break in the court session, he told the crowd: "The authorities are confused, weak, and instead of fighting (Russian President Vladimir) Putin, they are trying to fight us."
The court eventually deferred a decision on whether to order Poroshenko's arrest after a session lasting nearly 12 hours. The hearing resumes on Wednesday.
On Friday, Zelenskiy's chief of staff batted away suggestions that Zelenskiy was behaving like Viktor Yanukovich, a former president who was backed by Russia and whose political rival Yulia Tymoshenko was jailed in a case the West condemned as politicised.
Ukraine has tried to rally Western support in its stand-off with Moscow after sounding the alarm about a build-up of tens of thousands of Russian troops near its borders.
A confectionary tycoon, Poroshenko was elected as head of a pro-Western government after the Maidan street protests that ousted Yanukovich in 2014.
Prosecutors asked a Ukrainian court in December to arrest Poroshenko.
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