Saakashvili, who was also a regional governor in Ukraine, dramatically fell out with the country's President Poroshenko...
Saakashvili, who was also a regional governor in Ukraine, dramatically fell out with the country's President Poroshenko over accusations that Kiev was failing to make good on the fight against corruption and was stripped of his Ukranian passport.
In September, he defied the Ukrainian authorities and forced his way back into the conflict-riven country.
"Poroshenko is a thief, Poroshenko is a traitor to the people of Ukraine," Saakashvili said from the roof, a video broadcast on Ukrainian television showed.
"I call on all decent Ukrainians to take to the streets and drive out these thieves... I call on Ukrainians not to allow me to be abducted, I appeal to the people of Kiev not to allow lawlessness."
The Ukrainian Security Service (SBU) said Saakashvili, 49, had been arrested on charges of assisting criminal organisations.
The entrance to the building was blocked by dozens of armed police, some from the security service, after Saakashvili was led down from the roof and into a police van, an AFP journalist at the scene said.
More than a hundred supporters had blocked the vehicle in a street not far from where the arrest took place for more than an hour.
The security service said Saakashvili had been informed of the charges in the presence of his lawyer and would be questioned by investigators.
"This morning in the Ukrainian capital Kiev, my husband was taken by government troops from his home and moved by force into a mini-van that was subsequently blocked by hundreds of his supporters, growing already into the thousands," Saakashvili's wife Sandra Roelofs said on her Facebook page.
"Time will show where this leads to, we pray it will stay peaceful!"
President Vladimir Putin's spokesman said it was "once again surprised by Kiev and once again surprised by Saakashvili".
Asked to comment on the situation, Putin spokesman Dmitry Peskov said, "We are not used to responding to the statements of people sitting on roofs."
Saakashvili 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 key Odessa region on the Black Sea, before falling out with Poroshenko.
Saakashvili has led protests against the Kiev government since October after forcing his way back into the country from Poland.
The former Georgian president leads his own Ukrainian political party that hopes to grab seats from the president's increasingly unpopular one in polls set for autumn 2019.