According to Turkish authorities, an explosion in a crowded area of central Istanbul resulted in at least six fatalities and 81 injuries.
The explosion occurred on a retail street near Taksim Square at around 16:20 local time (13:20 GMT), according to Ali Yerlikaya, the governor of the Turkish city.
According to vice president Fuat Oktay, the explosion was likely the result of a female terrorist act.
President Recep Tayyip Erdogan said the perpetrators would be punished.
BBC correspondent Orla Guerin, who is in the area, said there was a heavy police presence around Istiklal Street, which had been cordoned off.
Eyewitness Cemal Denizci was about 50m (54 yards) away when he heard the explosion. “I saw three or four people on the ground,” he told AFP.
“People were running in panic… There was black smoke. The noise was so strong, almost deafening,” he said.