An explosion near a metro station Tuesday in Istanbul, Turkey, injured at least five people and brought the transport system to a halt.
A local official in Istanbul said the explosion in the evening rush hour at an overpass near the city's Bayrampasa station was caused by a pipe bomb.
Atilla Aydiner, the mayor of Istanbul's Bayrampasa district, also said a number of vehicles were damaged in the blast, including a bus and a car.
Authorities deployed riot police to secure the perimeter of the site in case of a second blast.
No claim of responsibility has been made.
Earlier, Istanbul's governor, Vasip Sahin, said the cause of the blast was not known, and authorities were assessing every possibility.
Some media reports said the explosion might have been caused by a bomb, and other reports said it could be linked to an electrical transformer.
Ambulances were dispatched to the scene, as commuters were helped to safety, reports said.
Turkey is currently on alert for attacks after more than 100 people were killed on October 10 when two suicide bombers detonated themselves in the middle of a crowd of peace activists in the capital, Ankara, in what is considered the worst attack in modern Turkey's history. One of the bombers is suspected of links to the Islamic State group.