The mayor was arrested as part of a corruption investigation, the report said.
Imamoglu's testimony at the prosecutor's office as part of the investigation was completed yesterday.
Imamoglu was detained on Wednesday. The opposition politician himself said that he "does not intend to give in" to pressure from the authorities. The detention of Istanbul's mayor has sparked mass protests, despite authorities tightening security and banning any rallies or demonstrations in the city through Sunday.
Opposition views the developments as an attempt to remove one of President Recep Tayyip Erdogan's rivals from the 2028 presidential election.