In December, Saakashvili climbed on the roof of a building in the Ukrainian capital, Kiev, as armed police stormed his apartment.
He was taken by men in green camouflage and his whereabouts are unknown, Reuters quoted his spokeswoman as saying.
"Whatever Poroshenko does, I am a Ukrainian politician. and I will remain a Ukrainian politician and fight to the end against corruption and the corrupt oligarchy in Ukraine and in Georgia, " Mr Saakashvili said in Warsaw on Tuesday.
Saakashvili was stripped of his Ukrainian citizenship when he was out of the country a year ago but returned in September, facing what he calls politically motivated extradition.
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The detention was carried out inside the restaurant Sulguni, which serves Georgian dishes and is situated nearby the office of the Movement of New Forces, an opposition political party founded by Saakashvili a year ago.
Mikheil Saakashvili, who spoke to Rustavi 2 TV from Warsaw, said the Ukrainian authorities "kidnapped" him through use of force, and "expelled" him from the country on a private plane.
"Mikhail Saakashvili has been received on Poland's territory", the statement reads.
The readmission was the result of the previously accepted application submitted by the Migration Service of Ukraine to the Commander-in-Chief of the Border Guard service of Poland.
Saakashvili said that he did not intend to seek asylum in Poland, and vowed to eventually return to Ukraine.
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Saakashvili was a key figure in the 2003 Rose Revolution protests in Georgia that ousted the country's president.
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. He had previously been deprived of his Georgian citizenship.
Ukrainian authorities accused Saakashvili of abetting an alleged "criminal group" led by Yanukovych, and claim the protests he has led are part of a Russian plot against the government in Kyiv.
"This is not a president and not a man".
Saakashvili is wanted in Georgia on embezzlement charges, which he says are politically motivated.
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