Turkey‘s Kandilli Observatory estimates that 500 to 1,000 people have died in the natural disaster, according to local media reports.
Deputy Prime Minister Besir Atalay told reporters some 10 buildings had collapsed in Van city and around 25-30 buildings had fallen in the nearby district of Ercis.
Television pictures showed damaged buildings, vehicles crushed under falling masonry, and panicked residents wandering in the streets.
Terrified people spilled on to the streets of Van in panic as rescue workers and residents using their bare hands and shovels struggled to evacuate people believed to be trapped under collapsed buildings.
Turkish media said phone lines and electricity had been cut off. Prime Minister Tayyip Erdogan will head to Van to see the damage, according to reports.
Aftershocks continued after the initial quake, whose epicenter was at the village of Tabanli, north of Van city.
Ercis Mayor Zulfikar Arapoglu said many people had died and several buildings had collapsed, and that medical aid was urgently needed.
The US Geological Survey originally reported that the magnitude of the earthquake was 7.6, but the Kandilli Observatory and Research Institute later estimated the strength at 6.6.
The institute said the earthquake struck shortly before 11am UK time and was three miles deep.