The huge blast caused panic close to the metro station in the Bayrampasa district of Istanbul and injured at least six people.

"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 dispatched to the scene, as commuters were helped to safety, reports said.

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.

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