The explosion hit Istiklal Street, a wide boulevard closed to traffic that is lined with international stores and shopping centres, just a few hundred metres from an area where police buses are usually parked, a Reuters witness said.
Armed police sealed off the main shopping street where half a dozen ambulances had gathered. Forensic teams in white suits were scouring the area for evidence.
Istiklal Street, usually thronged with shoppers on weekends, was quieter than normal before the blast, as more people are staying home after a series of deadly bomb blasts.
A suicide car bombing in the capital Ankara killed 37 people this month. A similar bombing in Ankara last month killed 29 people. A Kurdish militant group has claimed responsibility for both of those bombings.
In January, a suicide bomber killed around 10 people, most of them German tourists, in Istanbul's historic heart, an attack the government blamed on Islamic State
