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A strong explosion of unknown origin shook the busy shopping street of Istiklal in the city of on Sunday, leaving four people dead and wounding dozens more, officials said.

Police cordoned off the area, where crowds were dense on Sunday afternoon, and helicopters were flying over the city centre as sirens were sounding.

"I was 50–55 metres [yards] away, suddenly there was the noise of an explosion. I saw three or four people on the ground," witness Cemal Denizci, 57, told AFP.

"People were running in panic. The noise was huge. There was black smoke. The noise was so strong, almost deafening," he said.

Parents swept their children up into their arms as they fled the area.

Istanbul governor Ali Yerlikaya tweeted that four people died and 38 were wounded according to preliminary information.

Authorities have given no indication of what caused the explosion.

According to an AFP video journalist on the scene, the police have established a large security cordon to prevent access to the damaged area for fear of a second explosion.

A massive deployment of security forces equally barred all entrances, while a heavy deployment of rescue workers and police were visible.

The explosion occurred shortly after 4pm (1300 GMT) in the famous Istiklal shopping street which is popular with locals and tourists.

According to images posted on social media at the time of the explosion, it was accompanied by flames and immediately triggered panic, with people running in all directions.

A large black crater was also visible in those images, as well as several bodies lying on the ground nearby.

Istiklal Street had already been hit in the past during a campaign of attacks in 2015 and 2016 that targeted Istanbul.

Claimed by the , those attacks killed nearly 500 people and injured more than 2,000.