Five 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, spoke on live television after the explosion hit the industrial and residential area near the height of evening rush hour.
An Istanbul police source told NBC News that evidence of a small hand-made bomb had been found outside the entrance of the metro station.
Initial reports said that least one person was killed in the incident, and the explosion was first believed to have been from a power transformer, Turkey's Haberturk reported.
A municipal official told Reuters that train operations had been suspended at the Bayrampasa metro station after a loud noise was heard. The Associated Press reported that subway services throughout Istanbul had been halted on the entire network.
Dogan news agency reported that many ambulances were heard rushing to the area, which is a residential and industrial neighbourhood on the European side of the city.
Photographs on social media showed what appeared to be dozens of people walking alongside overground train tracks after trains had been halted.
Tuesday's blast was much smaller than the ones in Ankara and the town of Suruc near the Syrian border, which are believed to have been carried out by Islamic State militants.
Turkey, a NATO member, has carried out air strikes against the Islamist insurgents in neighbouring Syria as part of the U.S.-led coalition fighting the jihadist movement.
It also faces security threats from the outlawed Kurdistan Workers Party (PKK), whose militants often attack police and security in Turkey's mainly Kurdish southeast, and the far-left Revolutionary People's Liberation Army-Front (DHKP-C).
—NBC News, The Associated Press and Reuters contributed to this report.