"They have done it against all the laws, what is happening here!" But while perhaps not a threat as a direct rival, Saakashvili could prove to be an effective weapon against Poroshenko for powerful opposition figures like Yulia Tymoshenko, who was with him at the border on Sunday.
Late on Sunday, a group of Saakashvili's supporters broke through the line of security forces guarding the border and literally pulled Saakashvili from neutral territory between Poland and Ukraine onto Ukrainian territory.
Saakashvili had planned on taking an afternoon train from the southeastern Polish city of Przemyśl to Kyiv, but an onboard announcement said the train would not leave the station while carrying "a person who is not allowed entry to Ukraine". As Poroshenko has revoked that citizenship, Saakashvili is technically stateless, though he contends that the revocation is illegal because worldwide law prohibits stripping the citizenship of a person who would then become stateless.
Also with Saakashvili was Mustafa Nayyem, who was active in protests in Ukraine in 2013-14 that drove then pro-Russia president, Viktor Yanukovych, out of the country.
Georgia on December 4 stripped former leader and reformer Mikheil Saakashvili of his citizenship as he had acquired a Ukrainian passport to serve as governor of the strategic Odessa region.
Poroshenko's office said Saakashvili had failed to deliver change as governor, adding his Ukrainian citizenship was withdrawn because he allegedly put false information on his registration form. He said that he was not under investigation in Ukraine or overseas, while in fact he was being investigated in Georgia for various charges.
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Saakashvili is an outspoken critic of President Petro Poroshenko, who quit his governorship in November 2016 citing corruption and a lack of action by Poroshenko's administration to tackle it.
Saakashvili denies the accusations branding them a political witch hunt.
Saakashvili and a small crowd of supporters on Sunday shoved their way through a line of guards at the Ukrainian border, making good on the politician's vow to return to the country that had stripped him of his Ukrainian citizenship.
But a crowd of supporters, who chanted "Misha, Misha" - a diminutive of his name - forced their way into Ukraine with Saakashvili, who now risks extradition to his native Georgia.
"We see a roll-back of reforms in Ukraine, we see a crackdown on anti-corruption activities in Ukraine". Several border guards were injured in the incident, Ukrainian media reports.
While in Warsaw on Friday, Saakashvili asserted that Ukraine was witnessing a roll-back of reforms and efforts to stem corruption, reiterating that he wanted to get back into politics.
US Open tennis 2017: Live updates, results from Nadal vs. Anderson final
If past experience is at all predictive of future success, Nadal should make fairly quick work of Anderson on Sunday afternoon. The Big Four of Nadal, Federer, Novak Djokovic and Andy Murray has combined to win 45 of the last 50 major singles titles.