Four casualties were initially reported.
The explosion occurred on Istiklal Street, near Taksim Square, one of Turkey's most popular tourist attractions, in the Beyoglu district. The Consulate General of Russia is located on this street.
The cause of the explosion is currently under investigation. Dozens of ambulances, firefighters and rescuers were sent to the scene. The entire area is now cordoned off by law enforcement officers, people are being evacuated.
Turkish President Recep Tayyip Erdogan called the incident a "bomb attack".