Passengers walk at a metro station in Istanbul.

Six people were injured when a pipe bomb exploded on an overpass near an Istanbul metro station on Tuesday, the district mayor told A Haber television.

Atilla Aydiner, the mayor of the Bayrampasa district on the European side of the city, was speaking on live television after the explosion hit the industrial and residential area near the height of evening rush hour.

Initially, the origin of the blast, which brought the transport system in Turkey’s largest city to a halt, was unknown. Some earlier reports said the cause could be linked to an electrical transformer.

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.

Large numbers of ambulances were dispatched 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.

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