Two of the injured were in a critical condition, police spokesman Imran Shaukat said. The bomb appeared to have been planted on a motorbike.
It was not immediately clear who or what was the target of the blast in the Aisha Manzil area of Karachi, although a political rally was happening nearby. The explosion was heard several kilometres (miles) away.
Karachi, home to 18 million people, is the financial capital of Pakistan but has been plagued by Islamic militancy as well as violent crime.