An explosion caused by a homemade bomb rocked an Istanbul metro station on Tuesday, triggering panic in the evening rush hour and wounding six people.

The local mayor and the state-run Anatolia news agency confirmed a bomb caused the blast, which came at a time of growing jitters over security in Istanbul and the risk of a militant attack in Turkey's largest city.

The blast, which was heard in several areas of the city, hit an overpass close to the metro station in the Bayrampasa district of Istanbul.

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

Security sources were also quoted by the Dogan news agency as saying the blast appeared to have been caused by a homemade bomb.

The metro system was brought entirely to a halt after the blast, the municipality said, although normal service was gradually restored.

There was no immediate claim of responsibility for the explosion.

Turkey is on alert for attacks after 103 people were killed on Oct 10 when two suicide bombers ripped through a crowd of peace activists in the capital, Ankara, the worst attack in modern Turkey's history.