Saakashvili was arrested at a restaurant by several men in military uniforms on Monday, according to his spokesperson and two videos posted on his Facebook page.
“Unknown people in masks seized Mikheil Saakashvili and took him away … The kidnappers were in three white minivans,” a post read.
Oleh Slobodyan, a spokesman for the Ukrainian border service, said Saakashvili entered the country illegally in September from Poland after being stripped of his citizenship.
“This person was on Ukrainian territory illegally and, therefore, in compliance with all legal procedures, he was returned to the country from where he arrived,” Slobodyan said.
The border service refused to comment.
Saakashvili, a former Georgian president and current leader of an opposition party in Ukraine, is wanted in Georgia on four charges, including abuse of office – and faces separate criminal charges in Ukraine.
Ukrainian prosecutors accuse Saakashvili of having assisted a criminal organisation, a charge he claims is a politically motivated attempt to undermine his campaign to win the Ukrainian presidency, from incumbent Petro Poroshenko.
On January 5, a Ukrainian court rejected Saakashvili’s request for protection from extradition, denying him the status of a person in need of sanctuary.
Once allies, Saakashvili turned into an outspoken critic of Poroshenko since resigning as governor of the Black Sea Odessa region in November 2016, 18 months after assuming the post, claiming corruption was stifling his work.
In an interview, the politician said he wants to modernise Ukraine.
“I stand up for the idea of modernised, democratic, European Ukraine with an economy free of oligarchs [and] free of corruption because corruption is killing the economy,” he told Al Jazeera.
He added he thinks Ukrainian prosecutors have no evidence against him.
“The reason why they want to get rid of me is that they don’t have a real case against me, [and] because I crossed the path of oligarchs that want to further plunder the country,” he said.
“[Ukraine] is potentially the wealthiest country in Europe because of resources and right now it’s the poorest country in Europe, GDP-wise, because seven oligarchs hold hostage the entire wealth of the country and they regard me as their main enemy.”