According to Mousavi Websites: kaleme.org:
Mir-Hossein, Karoubi and their spouces have been arrested and are in Heshmaiteh (a Military) prison. Witnesses said, on midnight of last Thursday, security forces entered the parking of Karoubi residence and minutes later left with him in custody. Mousavi’s daugher have also stated, they have been unable to contact their parents in their residence (denied access), and are now certain they have been moved to the Heshmatieh priosn.
The order for their arrest according to Karoubi’s spokesman have come directly from Khamaneh-ee.