"Although it is too early to confirm 100%, initial reports and information from the governor's office show (the blast) smell like terror," Erdogan said at a press conference in Istanbul before heading to Indonesia for the G20 summit.
"It looks like a single woman had a role in it," added the president, noting a full-scale investigation was under way, Xinhua news agency reported.
Earlier, reacting to the blast, Erdogan said the security and health units were sent to the scene, and the wounded were transferred to the surrounding hospitals.
He said efforts to take over Turkey through terrorism would not work.
India conveyed its condolences to the Turkish government and people.
In a tweet, India's External Affairs Ministry spokesperson Arindam Bagchi said: "India conveys its deepest condolences to the Government and people of Turkiye on the tragic loss of lives in the blast that took place in Istanbul today. Our sympathies are also with those who sustained injuries. We wish them a speedy recovery."
An explosion hit the busy pedestrian Istiklal avenue on the European side of the city at 4.20pm local time, killing at least six people and wounding 53 others.
The blast caused panic in the city, particularly in the historic Beyoglu district where loud the sound rippled through.
A large number of health personnel arrived at the scene to tend to the wounded.
The police are on high alert.