Saakashvili later said police had searched the wrong tent at the camp, continuing a surreal hide-and-seek game between him and Ukrainian law enforcement ever since he barged across the border from Poland three months ago.
A statement posted on his official Facebook page called for a protest at the detention center.
She said the work of NABU and the Special Anti-Corruption Prosecutor's Office (SAPO) - a partner agency that is also under attack - was "crucial to achieving stronger and equitable growth".
"I assured the President that the IMF stands ready to continue to support Ukraine, along with other worldwide partners, in the fight against corruption and encouraged the authorities to accelerate the implementation of reforms", she said.
He was later freed from police custody by a crowd of his supporters.
Saakashvili's extraordinary energy and strong charisma took him to the forefront of the 2003 Rose Revolution that forced out the government in his native Georgia. Saakashvili then re-entered the country illegally and staged multiple rallies in the capital demanding the president be impeached.
Saaksahvili became a regional governor in Ukraine at Poroshenko's invitation, But he later turned on his one-time patron, accusing the Ukrainian leader of corruption.
In a letter sent to his supporters Saakashvili encouraged the Ukrainian people to join the march against President Petro Poroshenko.
Law enforcement officers searched Saakashvili's apartment in Kyiv on December 5, dragged him off the roof, and bundled him into a vehicle. The 49-year-old is facing the threat of possible extradition to Georgia, where he is wanted on criminal charges he says were trumped up for political reasons.