At least 20 aftershocks followed, some hours later during daylight, the strongest measuring 6.6, Turkish authorities said
President Recep Tayyip Erdogan on Sunday condemned the "vile attack" that ripped through central Istanbul, and which he said killed six people and wounded dozens of others.
"The relevant units of our state are working to find the perpetrators... behind this vile attack," Erdogan told a televised press conference. He also said initial signs pointed to a "terror" attack in the explosion in Istanbul which wounded 53 others.
"It might be wrong if we say for sure that this is terror but according to first signs... there is a smell of terror there," Erdogan said. He added, "The attempt to take over Turkey and the Turkish nation through terrorism will not reach its goal today or tomorrow."
"Our people can be rest assured that the culprits behind the attack will be punished as they deserve," he said, adding that initial information suggested "a woman played a part" in it.
The explosion occurred at around 4:20 pm (1320 GMT) in the famous Istiklal shopping street which is popular with locals and tourists.
Ambulances raced to the scene on the packed Istiklal Avenue, which police had quickly cordoned off. The area, in the Beyoglu district of Turkey's largest city, had been crowded as usual at the weekend with shoppers, tourists and families.
Video footage obtained by Reuters showed the moment the blast occurred in the centre of the avenue, sending debris into the air and leaving several people lying on the ground.
Nobody has claimed responsibility for the blast, but Istanbul and other Turkish cities have been targeted in the past by Kurdish separatists, Daesh and other groups.
"When I heard the explosion, I was petrified, people froze, looking at each other. Then people started running away. What else can you do," said Mehmet Akus, 45, a worker in a restaurant on Istiklal.
"My relatives called me, they know I work on Istiklal. I reassured them," he told Reuters.
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A helicopter flew above the scene of the blast and a number of ambulances were parked in nearby Taksim Square.
The Kasimpasa police station said all crews were at the scene but gave no further details.
Local media said the Istanbul chief public prosecutor's office had launched an investigation into the blast.
The Turkish Red Crescent said blood was being transferred to nearby hospitals.
If confirmed, it would be the first major bomb blast in Istanbul in several years.
Twin bombings outside an Istanbul soccer stadium in December 2016 killed 38 people and wounded 155 in an attack claimed by an offshoot of the militant Kurdistan Workers Party (PKK).
At least 20 aftershocks followed, some hours later during daylight, the strongest measuring 6.6, Turkish authorities said
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