The huge blast caused panic close to the metro station in the Bayrampasa district of Istanbul and injured at least six people, NTV television said. NTV said it may have been caused by a bomb while other reports said the cause could be linked to an electrical transformer.
"The cause of the explosion is not clear. We are investigating all possibilities," Istanbul governor Vasip Sahin was quoted as saying by the state-run Anatolia news agency.
Large numbers of ambulances were despatched to the scene, as commuters were helped to safety, reports said.
"Metro services have been halted after a sound similar to an explosion was heard, its cause remains unknown," the Istanbul municipality said in a statement.
Turkey is currently on alert for attacks after 103 people were killed on October 10 when two suicide bombers ripped through a crowd of peace activists in the capital Ankara, the worst attack in modern Turkey's history.