Amid shouts of "victory" and "glory to Ukraine", Saakashvili returned to Ukraine despite being stripped of Ukrainian citizenship by his one-time ally, Ukrainian President Petro Poroshenko, and facing possible arrest and deportation.
Kremlin Spokesman Dmitry Peskov has slammed Ukraine's criminal probe launched against Georgian ex-president and former Odessa Region's governor Mikhail Saakashvili over the illegal border crossing as "political circus", TASS reports. The former head of state of Georgia is accused of abuse of power while he was the governor of Odessa.
Supporters who accompanied him to the crossing point included former Ukrainian Prime Minister Yulia Tymoshenko and Mustafa Nayyem, a lawmaker who was a key figure in the 2013-14 protests that drove Russia-friendly Ukrainian President Viktor Yanukovych out of the country. The woman in charge of the Ukrainian train said she had been ordered by the authorities - she declined to specify whether Polish or Ukrainian - to stop the train leaving until Saakashvili got off.
"But instead, he committed a crime - because the state border must not be violated", Poroshenko said.
Ukrainian officials say he entered illegally, and 16 border guards and National guardsmen were injured.
The return carries risks for Saakashvili, who is stateless.More news: Trump addresses USA state governors about Hurricane Erma
But he quit in November 2016 amid a dramatic falling out with Poroshenko, who stripped him of his Ukrainian citizenship in July while he was out of the country.
The Ukrainian border service said in a Facebook post that the crowd broke through a checkpoint and that fighting broke out when guards tried to block Saakashvili's supporters. He said that he was not under investigation in Ukraine or overseas, while in fact he was being investigated in Georgia for various charges.
"I want to say that this is the beginning of my fight", he said, adding that he had a plan to turn Ukraine's economy and politics around and was determined to tour the country.
Georgian authorities have asked Kiev to extradite Mr Saakashvili, saying he is a defendant in four criminal cases. "Misha!" (Saakashvili's nickname) and waving yellow-and-blue Ukrainian flags, the now stateless politician made his way to the Shehyny checkpoint, which was closed by Kiev's authorities earlier Sunday, according to media reports.
Kiev justified the move by claiming that Saakashvili had provided "inaccurate information" in his citizenship application.