On Friday, two court officials and a dozen police officers arrested Bryant from his residence before he was taken to the prison compound.
He told reporters that he did not know how long he would be in detention.
Ellen Johnson-Sirleaf, Liberia's current president, has vowed zero tolerance for corruption as she tries to return the country to its former prosperity.
The Bryant administration, which grouped all the factions from the civil war, was tasked with guiding Liberia to elections after Charles Taylor, an ex-dictator, now on trial in The Hague for war crimes, went into exile.
Bryant has denied the findings of the ECOWAS report and has warned that any prosecutions could prompt a constitutional crisis in the mineral-rich state .