Georgia's former president, who later served as governor of Ukraine's Odessa region, has pledged to unite the opposition against his former ally and job-giver Ukrainian President Petro Poroshenko.

Mikheil Saakashvili said on Monday he was not seeking the presidency for himself, but wanted to see Poroshenko voted out at the next election in 2019.

The comments came a day after Saakashvili forced his way past border guards into his adopted homeland with hundreds of supporters in protest against the withdrawal of his Ukrainian citizenship by the government.

"I am fighting against rampant corruption, against the fact that oligarchs are in full control of Ukraine again, against the fact that Maidan has been betrayed," Saakashvili said at a press conference in the western city of Lviv, referring to the anti-government uprising that saw pro-Russian President Viktor Yanukovich toppled in favour of the Western path for the country's development.

Saakashvili, who studied in Ukraine and speaks fluent Ukrainian, received Ukrainian citizenship in 2015 from Poroshenko when the president made him governor of Odessa.

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Their relationship dates back nearly three decades to when they were students at the same university in Kiev and their shared opposition to the Russian government brought them together as politicians.

But a bitter spat erupted in November 2016, when Saakashvili quit and started accusing Poroshenko of abetting corruption.

Poroshenko's office said Saakashvili had failed to deliver change as governor, adding his Ukrainian citizenship was withdrawn because he allegedly put false information on his registration form.

Saakashvili said the decision was politically motivated. It left him effectively stateless as he previously lost his Georgian citizenship because of the country's rules about dual nationality.

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Saakashvili divides opinion in Ukraine, which is at war with Russia-backed rebels in its east.

Supporters see him as a fearless crusader against corruption, but critics say there is little substance behind his blustery rhetoric. Polls show little support for his party in Ukraine, the Movement of New Forces.

Back home in Georgia, he started out as an internationally praised reformer but later he was criticised for monopolising power and exerting pressure on the judiciary.

He was president at the time of a five-day war with Russia in 2008, a conflict his critics argued was the result of his own miscalculations.

Kiev could leave Saakashvili alone or arrest him and possibly extradite him to Georgia, where the 49-year-old is wanted on criminal charges, which he says are politically motivated.