Türkiye is in one of the world's most active earthquake zones.
Düzce was one of the regions hit by a 7.4-magnitude earthquake in 1999 -- the worst to hit Türkiye in decades.
Experts have long warned a large quake could devastate Istanbul, which has allowed widespread building without safety precautions.
A magnitude-6.8 quake hit Elazığ in January 2020, killing more than 40 people.
And in October that year, a magnitude-7.0 quake hit the Aegean Sea, killing 114 people and wounding more than 1,000.
The earthquake levelled dozens of buildings across major cities of southern Türkiye as well as neighbouring Syria, a country gripped by more than a decade of violence that has killed hundreds of thousands and displaced millions of people.
Images on Turkish television and social media showed rescuers digging through the rubble of levelled buildings in the city of Kahramanmaras and neighbouring Gaziantep.
A fire lit up the night sky in one image from Kahramanmaraş, although its origin remained unclear.
NTV television said buildings also crumbled in the cities of Adıyaman, Malatya and Diyarbakır.
CNN Turk television said the quake was also felt across parts of central Türkiye and the capital Ankara.
