Ukrainian prosecutors allege that Saakashvili cooperated with allies of former Ukrainian president Viktor Yanukovych, now in hiding in Russia, and took money from the Russians.
A hearing of the case of Mikheil Saakashvili, the leader of the Movement of New Forces party, is scheduled for 11.00 on Monday, December 11, his spokeswoman, Mariana Poshtar, has told Ukrinform.
Saakashvili, who was refusing food to protest the arrest, said that the first thing he now wants is to get back home and eat.
Saakashvili's supporters marched across downtown Kiev, chanting "Down with the Gang!" and "Impeachment!"
Saakashvili could be extradited to Georgia where he is wanted on corruption charges.
Saakashvili's wife, Sandra Elisabeth Roelofs, said Ukrainian authorities "crossed a red line" by arresting him.
The demonstrators smashed the windows of a candy store belonging to Poroshenko's company, Roshen. Saakashvili was elected the next year to replace him.
Protester Valery Shapovalov carried a big poster showing Poroshenko behind bars. "Bees can't oppose honey".
A pariah of the Kremlin after Georgia fought a short war with Russian Federation in 2008, Saakashvili was a vocal champion of the three-month uprising in Kiev that toppled the Moscow-backed Yanukovych government in 2014 and turned Ukraine towards a pro-EU course.
The 49-year-old politician served two terms as Georgia's president from 2004 to 2013.
He was later stripped of his Ukrainian citizenship, becoming stateless as he had lost his Georgian citizenship to gain the Ukrainian one. Saakashvili had supported the revolution in Ukraine that ousted Yanukovych; Poroshenko later named him governor of Odessa province.
He fled to Ukraine in 2014 and was granted Ukrainian citizenship.
He forced his way into Ukraine in September, barging across the border with Poland with the help of a crowd of supporters.