At least five people injured as suspected bomb blast rocks Istanbul metro system

A suspected pipe bomb explosion has hit the Istanbul metro, causing injuries and bringing the transport system in Turkey's largest city to a halt.

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

"Five of our citizens were injured when a pipe bomb left on barriers on the overpass exploded," Atilla Aydiner, the district mayor, told A Haber television. 

Video footage from the Dogan news agency showed large sparks shooting out like fireworks from the overpass, lighting up the sky near the height of evening rush hour.

Security sources were also quoted by Dogan as saying the blast appeared to have been caused by a home-made bomb.

There are unconfirmed reports one person was killed in the blast.

Other local media reports indicated the explosion 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.

The metro system was brought entirely to a halt after the blast, although reports suggested that normal service was now being restored.

"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.

Turkey is also waging an all out assault on the outlawed Kurdistan Workers Party (PKK) who have staged dozens of deadly attacks against members of the security forces but largely in the southeast of the country.