KIEV (AFP) - Former Georgian President Mikheil Saakashvili vowed on Monday (Sept 11) to reclaim his place in Ukrainian politics after he defied the authorities and forced his way back into the conflict-riven country.

Mr Saakashvili, a former regional governor of Ukraine's key Odessa region, and hundreds of his supporters on Sunday barged their way past guards at a border crossing with Poland.

Mr Saakashvili's dramatic return is the latest twist in a bitter feud with President Petro Poroshenko, who stripped him of Ukrainian citizenship in July.

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 international image.

Speaking at a news conference in the western city of Lviv on Monday, Mr Saakashvili insisted he aimed to restore his Ukrainian citizenship and get back into politics.

"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. "We should have democracy in our country and should not have the diktat of the oligarchs."

Mr Poroshenko cancelled Mr Saakashvili's passport after the two had a major falling out over the ex-Georgian leader's accusations that Kiev was failing to make good on the fight against corruption.

That move left the charismatic pro-Western politician stateless as he had earlier been stripped of his citizenship in his homeland Georgia.

The failure to stop Mr Saakashvili from returning was an embarrassment for Kiev and another headache for a leader fighting a Russian-backed insurgency and trying to restart a struggling economy.

"A crime has been committed," Mr Poroshenko said earlier on Monday. "There should be an absolutely unequivocal legal, judicial responsibility."

Mr Poroshenko said he saw no difference between pro-Russian rebels breaching Ukraine's border in the east and "politicos" making their re-entry from the west.

Interior Minister Arsen Avakov called the border breach "an attack on the state's basic institutions".

Mr Saakashvili, 49, is credited with pushing through pro-Western reforms in his native Georgia, which he led from 2004 to 2013 after rising to power during the so-called Rose Revolution in 2003.

In the wake of the pro-Western revolution in Kiev, he moved to Ukraine in 2015 to work for the country's authorities as governor of the Odessa region on the Black Sea.

Some analysts say Mr Saakashvili is seeking to rouse supporters and eventually unseat Mr Poroshenko.

"Saakashvili's return to Ukrainian politics was triumphant albeit scandalous," Mr Vadym Karasyov, director of the Kiev-based Institute of Global Strategies, said.

As of this May, the approval rating of Mr Saakashvili's party, the Movement of New Forces, stood at 1.1 per cent, according to a local pollster.

Mr Saakashvili was accompanied by former Ukrainian prime minister Yulia Tymoshenko, who some critics say is hoping to ride the firebrand politician's coattails back to power.

Video footage showing chaotic scenes of Mr Saakashvili's supporters overpowering border guards went viral, with pro-Western lawmaker Svitlana Zalishchuk labelling it a "circus".

Speaking in an interview to his 11-year-old son Nikoloz posted online, Mr Saakashvili said people essentially had carried him through the border.

"I was tremendously impressed," he said.

Ukrainian authorities had blocked a Kiev-bound train carrying Mr Saakashvili earlier on Sunday. He eventually got off the train and took a bus to Medyka, where he successfully crossed the border.

The interior ministry has said that 11 policemen and five border guards were injured in clashes with Mr Saakashvili's supporters.

Some commentators decried the political stand-off.

"Yesterday and today millions of Ukrainian nationals are feeling ashamed of their authorities, of the opposition and - at the end of the day - themselves," wrote political observer Yevhen Kuzmenko.

Georgian authorities have asked Kiev to extradite Mr Saakashvili, saying he is a defendant in four criminal cases.

Mr Saakashvili has denied the accusations, branding them a political witch hunt.