The blast occurred on a main road near the city centre used by buses carrying employees of the local police force to and from work, the sources said.

A passer-by and the driver and a passenger of a municipal vehicle, which was damaged by the blast, were injured.

There were no police buses in the vicinity at the time of the explosion.

The exact cause of the blast was not immediately clear but police were investigating whether it was caused by a bomb, the sources said.

The blast followed a wave of Kurdish riots last week that began here and spilled over to other regions, claiming 15 lives in all.

Two bomb attacks claimed by a radical Kurdish group in reprisal for the police response to the riots hit Istanbul over the past week, killing one and injuring several others.