The former Ukrainian president, Mikhail Saakashvili, has been barred from crossing into Poland by the Ukrainian authorities while trying to enter the country to regain Ukrainian citizenship, which was withdrawn from him by Ukrainian President Petro Porchenko.
Mr Poroshenko vowed to punish those responsible for Mr Saakashvili's breach of the border, while observers said they feared the ugly political spat would hit Ukraine's global image.
Now, Saakashvili wants to return to challenge that decision in court and get back into politics.
"Poroshenko took all passengers of this train as hostages and is telling everyone that no-one will go anywhere", Saakashvili told reporters.
"I crossed the Polish border with a valid Ukrainian passport, the Poles stamped it", Saakashvili told reporters according to Ukrainian outlet Unian.
"But instead, he committed a crime - because the state border must not be violated", Poroshenko said. He has been known as a pro-Western, pro-NATO political leader in both Georgia and Ukraine.
Saakashvili's relationship with Poroshenko dates back almost three decades to when they were students at the same university in Kiev and their shared opposition to the Kremlin later brought them together as politicians. According to him, he left his Ukrainian passport in a bus on the border with Ukraine, but then "the police came and stole it".
"A forceful breakthrough of the Ukrainian border is a crime and those who were involved in the organization of this breakthrough should be held responsible", Ukrainian Prime Minister Volodymyr Groysman wrote on Facebook today.
Mikheil Saakashvili said on Monday he was not seeking the presidency for himself, but wanted to see Poroshenko ousted at the next election in 2019. No need to forget that home country, Georgia, accused Saakashvili of abuse of power and misappropriation of property, now it has sent an extradition request for him to Ukraine.
Ukrainian law enforcement agencies said Sahakashvili and his supporters face a possible 5 year prison term.
Hundreds of Saakashvili supporters broke through a police block and burst into the neutral zone where they paraded victorious with Saakashvili into Ukraine, cheering his nickname "Misha" and "glory to Ukraine".
Prosecutor-General Yury Lutsenko said late on September 10 that charges would be pursued against the people who helped organize Saakashvili's entry.
"It looks like [Poroshenko] is getting rid of a political opponent. I hope that this gentleman will have the same zeal to make his way to Georgia, as he tried to get to Ukraine", Poroshenko said.