Emergency workers battled to rescue people trapped in buildings in the city of Van and surrounding districts on the banks of Lake Van, near Turkey's border with Iran.
Some 10 buildings collapsed in Van city and about 25-30 buildings were brought to the ground in the nearby district of Erciş, Deputy Prime Minister Besir Atalay told reporters.
"We estimate around 1,000 buildings are damaged and our estimate is for hundreds of lives lost. It could be 500 or 1,000," Kandilli Observatory general manager Mustafa Erdik told a news conference.
Prime Minister Tayyip Erdogan was traveling to Van and the cabinet was expected to discuss the quake at a meeting called for Monday morning.
"A lot of buildings collapsed, many people were killed, but we don't know the number. We are waiting for emergency help, it's very urgent," Zulfukar Arapoglu, the mayor of Ercis stated.
"We need tents urgently and rescue teams. We don't have any ambulances, and we only have one hospital. We have many killed and injured," he said.
Turkey's Red Crescent said one of its local teams was helping to rescue people from a student residence in Erciş. It said it was sending tents, blankets and food to the region.
More than 20 aftershocks shook the area, further unsettling residents who ran out on the streets when the initial strong quake struck. Television pictures showed rooms shaking and furniture falling to the ground as people ran from one building.
Dozens of emergency workers and locals crawled over a multi-storey building in Van as they searched for any people trapped in side.
Elsewhere, vehicles lay crushed by falling masonry in the street while dazed-looking people wandered past.
Some 50 injured people had been taken to hospital in Van.
The Kandilli Observatory, which initially cited a magnitude of 6.6, said the earthquake struck at 1041 GMT (6:41 a.m. EDT) and was 5 km (3 miles) deep. The U.S. Geological Survey earlier reported that the magnitude was 7.6.
Turkish media said phone lines and electricity had been cut off. The quake's epicentre was at the village of Tabanli, 20 km north of Van city, Kandilli said.
In Hakkari, about 100 km (60 miles) south of Van, a building could be felt swaying for about 10 seconds during the quake but there was no immediate sign of casualties or damage in the town, about two and a half hours drive through the mountains from Van.
Major geological fault lines cross Turkey and small earthquakes are a near daily occurrence. Two large quakes in 1999 killed more than 20,000 people in northwest Turkey.
Two people were killed and 79 injured in May when an earthquake shook Simav in northwest Turkey.