Istanbul (Turkey) (AFP) - An explosion tore through a busy Istanbul shopping street on Sunday, killing six and wounding dozens in what Turkey’s president said bore the signs of a “terror” attack.

Police cordoned off an area around Istiklal, where there were dense crowds on Sunday afternoon, and helicopters flew over the city centre as sirens sounded.

“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,” he said.

President Recep Tayyip Erdogan condemned what he called a “vile attack”.

“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 told a press conference.

Turkey’s vice president Fuat Oktay said: “We believe that it is a terrorist act carried out by an attacker, whom we consider to be a woman, exploding the bomb”.

Justice minister Bekir Bozdag said: “A woman had been sitting on one of the benches for more than 40 minutes and then she got up.

“One or two minutes later, an explosion occurred,” he told A Haber television.

“There are two possibilities,” he said. “There’s either a mechanism placed in this bag and it explodes, or someone remotely explodes (it)”.

“All data on this woman are currently under scrutiny,” he added.

- Area targeted previously -

Authorities offered few details and nobody immediately claimed responsibility, but Turkish cities have been struck by Islamists and other groups in the past.

Istiklal Avenue was hit during a campaign of attacks in 2015-2016 that targeted Istanbul and other cities including the capital Ankara.

Those bombings were mostly blamed on the Islamic State group and outlawed Kurdish militants, and killed nearly 500 people and injured more than 2,000.

Sunday’s explosion occurred shortly after 4:00 pm (1300 GMT) in the famous shopping street which is popular with locals and tourists.

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

Several bodies were seen lying on the ground nearby in the images.

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

- Condemnation -

Istiklal, in the historic district of Beyoglu, is one of the most famous arteries of Istanbul. It is entirely pedestrianised for 1.4 kilometres (almost a mile).

Criss-crossed by an old tramway, lined with shops and restaurants, it sees large crowds at the weekend.

In the neighbouring district of Galata, many stores closed early while some passers-by, who came running from the site of the explosion, had tears in their eyes.

A massive deployment of security forces barred all entrances, and rescue workers and police could be seen.

Turkey’s radio and television watchdog, RTUK, placed a ban on broadcasters showing footage of the blast – a routine measure previously taken in the aftermath of terror attacks.

Access to social media was also restricted after the attack.

A reaction came quickly from Greece, which “unequivocally” condemned the blast and expressed condolences to the government and people of Turkey.

The United States also denounced it, with White House Press Secretary Karine Jean-Pierre saying: “We stand shoulder-to-shoulder with our NATO Ally Turkey in countering terrorism.”

French President Emmanuel Macron said in a message to the Turks: “We share your pain. We stand with you in the fight against terrorism”.

“Shaken by news of the despicable bombing in Istanbul targeting innocent civilians,” Israeli President Isaac Herzog tweeted in Turkish and English saying: “The whole world must stand united and firm against terror.”

Ukrainian President Volodymyr Zelensky tweeted also in Turkish: “The pain of the friendly Turkish people is our pain.”

EU Council President Charles Michel offered condolences to Turkey, tweeting: “My thoughts are with the victims & their families.”