Istanbul: Six people were killed and 53 others were wounded on Sunday when an explosion rocked a busy pedestrian street in central Istanbul in what Turkish President Tayyip Erdogan called a bomb attack that "smells like terrorism".

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 at 4.13 p.m. (1313 GMT), sending debris into the air and leaving several people lying on the ground, while others fled the scene.

"Efforts to defeat Turkey and the Turkish people through terrorism will fail today just as they did yesterday and as they will again tomorrow," Erdogan told a news conference in Istanbul.

"Our people can 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.

"It would be wrong to say this is undoubtedly a terrorist attack but the initial developments and initial intelligence from my governor is that it smells like terrorism," he added.

Nobody has claimed responsibility for the blast. But Istanbul and other Turkish cities have been targeted in the past by Kurdish separatists, Islamist militants and other groups, including in a series of attacks in 2015 and 2016.

'People froze'
A footage showed people attending to victims after the blast, and later investigators in white outfits collecting material from the scene, where pieces of a concrete planter were scattered on the avenue.

"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.

A helicopter flew above the scene and a number of ambulances were parked in nearby Taksim Square. 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).

Condemnations of the attack and condolences for the victims rolled in from several countries including Greece, Egypt, Ukraine, Azerbaijan and Pakistan.

European Council President Charles Michel said on Twitter he had sent condolences to victims after the "horrific news".