In a brief stand-off at border control after arriving on a flight from Warsaw, Poroshenko accused border guards of taking away his passport. He later emerged to a crowd of thousands of cheering and flag-waving supporters outside the airport.
Poroshenko’s return sets up a showdown with President Zelensky’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. He could face 15 years in prison if convicted. His party accused Zelensky of a reckless attempt to silence political opposition.
Zelensky’s administration says the prosecutors and judiciary are independent and accuses Poroshenko of thinking he is above the law.
“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 into his case, 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.”
Tetiana Sapyan, a spokeswoman for the State Bureau of Investigations (DBR), told a separate briefing that the DBR was apolitical.
On Friday, Zelensky’s chief of staff batted away suggestions that Zelensky was behaving like Viktor Yanukovych, a former president who was backed by Russia and whose political rival Yulia Tymoshenko was jailed in a case that Western countries widely condemned as politicized.
The row has raged as Ukraine tries to rally Western support in its standoff with Moscow after sounding the alarm about a buildup 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 Yanukovych in 2014.
Relations between Ukraine and Russia collapsed in 2014 after Russia annexed the Crimea peninsula and backed separatist fighters in a conflict in eastern Ukraine.
Zelensky 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.
Prosecutors asked a Ukrainian court in December to arrest Poroshenko, with the possibility of bail set at 1 billion hryvnia ($37 million).