"The third president of Georgia, Mikheil Saakashvili, has been detained, he has been transferred to a penitentiary institution", the prime minister said.
According to the prime minister, "law enforcement officers carried out the arrest procedure at the highest level".
"Our law enforcement officers had preliminary information about his movement from Ukraine to Georgia", Garibashvili said.
He did not specify where Saakashvili had been detained.
According to Georgian broadcaster Pirveli, he was presumably transferred to a prison in Tbilisi.
At the same time, Maria Barabash, the executive director of Saakashvili's Ukraine-based organisation Office for Simple Decisions and Results, said that she did not believe in the detention of Saakashvili in Georgia, seeing it as a provocation for him to go online.
Georgian authorities have been seeking Saakashvili's extradition since 2014, as they charged the politician with several felonies, such as abuse of power and the embezzlement of $5 million in state funds. In 2015, Saakashvili renounced his Georgian citizenship and became Ukrainian after he was appointed governor of Ukraine's Odessa region.