At least six people have been killed and 53 wounded in an explosion in a busy area of central Istanbul, Turkish authorities have said.
The blast happened at about 16:20 local time (13:20 GMT) on a shopping street in the Taksim Square area, the Turkish city's governor Ali Yerlikaya said.
It is unclear what caused the explosion, which appears to have been captured on social media videos.
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. Helicopters were circling overhead as ambulances went back and forth.
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.
The street - usually packed with shoppers - was previously targeted by a suicide bomber in 2016.