Footage posted online showed ambulances, fire engines and police at the scene on Istiklal Avenue – a typically crowded thoroughfare popular with tourists and locals and lined with shops and restaurants.
In one video, a loud bang could be heard and flames could be seen, as pedestrians turned and ran away. Social media users said shops were shuttered and the avenue closed down.
Turkish president Recep Tayyip Erdogan called the blast a “treacherous attack” and said its perpetrators would be punished.
Mr Erdogan said six people had been killed. Vice-president Fuat Oktay later updated the wounded toll to 81, with two people in a serious condition,
Mr Erdogan did not say who was behind the attack, but he said it had the “smell of terror”, without offering details. The Turkish leader said he was not absolutely certain yet that it was a terror incident.
He said investigations were ongoing by the police and the governor’s office, including reviewing footage of the area.
Turkey was hit by a string of bombings between 2015 and 2017 that left more than 500 civilians and security personnel dead.
Some of the attacks were perpetrated by the so-called Islamic State group, while others were executed by Kurdish militants who have led a decades-long insurgency against the Turkish state for increased autonomy or independence.
Turkey has been fighting the militants – known as the PKK and considered a terrorist organisation by Turkey, the US and the European Union – in the country’s south-east for years.
Following the string of attacks, Turkey launched cross-border military operations into Syria and northern Iraq against Kurdish militants, while also cracking down on Kurdish politicians, journalists and activists at home through broad terror laws that critics say are a way to silence dissent.
Five prosecutors were assigned to investigate the blast, the state-run Anadolu news agency reported.
Turkey’s media watchdog imposed temporary restrictions on reporting on the explosion – a move that bans the use of close-up videos and photos of the blast and its aftermath.
The Supreme Council of Radio and Television has imposed similar bans in the past, following attacks and accidents.