At least six people have died and 81 are injured after an explosion in Istanbul city center which officials have described as an apparent act of terrorism. 

Turkey’s president Recep Tayyip Erdogan said the explosion on the busy Istiklal Street around 4.20 pm local time was caused by a “heinous” bomb attack. He said the explosion “smells like terrorism”, although he said investigations were still underway.

The country’s Vice President Fuat Oktay later updated the wounded toll to 81 and also said it appeared to be a terrorist attack.

Officials said a woman may have played a part in the explosion, although nobody has claimed responsibility for the blast.

Authorities also said a government ministry worker and his young daughter were among the dead. Five people were in intensive care in the hospital, two of them in critical condition.

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

"Our people can rest assured that the culprits behind the attack will be punished as they deserve.

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

Footage posted online showed flames erupting and a loud bang, as pedestrians turned and ran away.