The Kandilli Observatory's seismology center said the quake was centered in the town of Cinarcik, about 30 miles (50 km) across the Sea of Marmara from Istanbul and occurred at 8:53 p.m. local time.
Many people left their homes in fear of stronger aftershocks, spending the night sleeping in parks.
Cinarcik was one of the communities devastated by two strong temblors that hit northwestern Turkey in 1999, killing 18,000 people.
