In a brief standoff at border control on Monday (January 17), he accused the border guards of taking away his passport. He later emerged to a crowd of flag-waving supporters outside the airport.
At a separate briefing, Ukrainian investigators said they had tried to serve a subpoena to Poroshenko, who refused to take the documents.
Poroshenko is being investigated for alleged treason linked to the financing of Russian-backed separatist fighters through illegal coal sales while in office in 2014-15.
Prosecutors in December asked a Ukrainian court to arrest Poroshenko, with the possibility of bail set at 1 billion hryvnia ($37 million).