At least six people have been killed and 53 injured in an explosion in the heart of the Turkish city of Istanbul on Sunday afternoon, according to the city’s governor.

Those who were injured are being treated, the governor, Ali Yerlikaya, added.

“We wish God’s mercy on those who lost their lives and a speedy recovery to the injured,” he tweeted.

Yerlikaya earlier confirmed that the explosion took place on Istiklal Street in Beyoglu Square.

“Our police, health, fire and AFAD teams were sent to the scene,” he said. “Developments will be shared with the public.”

Calling the explosion in Istanbul on Sunday afternoon an attack, Turkish President Recep Tayyip Erdogan said state authorities would work to identify those responsible for it.

“All the responsible figures will be identified and punished,” Erdogan said.

Erdogan added that he and his delegation would be departing shortly for the G20 summit in Bali, Indonesia, as planned.

An investigation has been launched into the explosion, the country’s official Anadolu news agency reported.

Istanbul’s Chief Public Prosecutor’s Office has begun the investigation, with five public prosecutors assigned to it, according to the agency.

The city’s criminal court issued a broadcast ban on all visual and audio news, as well on social media sites, related to the explosion, Anadolu added.

Local media reports and images from the area showed a large number of emergency vehicles in the aftermath of the blast, with at least one person receiving medical attention. Some people could be seen fleeing the scene in the pictures and the area was being cordoned off by security services.

The cause of the explosion is not known and CNN cannot independently verify whether there are any casualties.

One eyewitness, journalist Tariq Keblaoui, told CNN that several people could be seen lying on the ground following the blast on Istiklal Street.

Keblaoui said he was in a store on Istiklal Street when the explosion occurred about 10 meters ahead of him.

The extent of the injuries of those he saw was not clear but several people were bleeding from their legs and arms, he said.

Keblaoui said Istiklal Street, a popular tourist area, was heavily crowded on Sunday. Istiklal Street is one of the main streets leading to Taksim Square.

Istanbul Mayor Ekrem Imamoglu called for assistance for police and health worker teams as they respond to the explosion in the city.

“It is essential to assist our police and health teams regarding the explosion on Istiklal Street, and to avoid posts that may cause fear and panic. All relevant teams are in the region, we will provide healthy information,” he tweeted.