An explosion in a shopping district at the Turkish city of Istanbul injured at least seven people including a police officer on Thursday.

Police said they were still investigating the cause of the blast. The device was planted in a dustbin near a bus station.

The blast occurred a couple of weeks before the parliamentary election. Security forces are on edge as there has been a rise in militant activity in recent weeks related to a long-running Kurdish separatist insurgency.

It wasn't like gunfire. It was like an explosion, Tuncay Ozdemircan told Reuters Television.

When I looked out of the window I saw yellow and white smoke rising, and I thought that it was a bomb, he said.