A strong quake has shattered buildings near the Turkish city of Van, killing more than 200 people and trapping an unknown number under rubble.
Casualties are reported to be particularly high in the town of Ercis, close to the Iranian border, where dozens of buildings fell.
Reports spoke of thousands of residents running screaming in the streets.
Rescuers worked into the night to find survivors, some with shovels or their bare hands.
Turkey is particularly vulnerable to earthquakes because it sits on major geological fault lines.
Two earthquakes in 1999 with a magnitude of more than 7 killed almost 20,000 people in densely populated parts of the north-west of the country.
'One thousand buildings'
The earthquake struck at 13:41 (10:41 GMT) at a depth of 20km (12.4 miles), with its epicentre 16km north-east of Van in eastern Turkey, the US Geological Survey said.
It was followed by a series of powerful aftershocks, also centred north of Van, including two of magnitude 5.6 soon after the quake and one of 6.0 late on Sunday.
Prime Minister Recep Tayyip Erdogan visited the Van region and Ercis by helicopter.
Up to 80 buildings, including a dormitory, collapsed in the town about 60km north of Van, while 10 fell in Van itself.
Town mayor Zulfikar Arapoglu appealed for help. "We need urgent aid, we need medics," he said.
Ambulances, soldiers and rescue teams rushed to the town, a Reuters photographer reported from Ercis.
Survivors complained of a lack of heavy machinery to remove chunks of cement floors that had pancaked on to each other, the Associated Press reports.
Serious damage and casualties were also reported in the district of Celebibag, near Ercis.
"There are many people under the rubble," said the local mayor, Veysel Keser.
"People are in agony, we can hear their screams for help. We need urgent help."
The head of Turkey's seismology institute said hundreds of people could have been killed.
"We estimate around 1,000 buildings are damaged and our estimate is for hundreds of lives lost - it could be 500 or 1,000," said Mustafa Erdik, general manager of the Kandilli Observatory.
As evening fell, residents of Van and Ercis lit camp fires, preparing to spend the night outdoors.
Rescuers could be seen working by torchlight, using their hands and shovels.
Temperatures were expected to drop to near freezing overnight. The quake cut electricity and telephone lines and the authorities in some areas have cut gas to avoid the risk of fire.
The BBC's David O'Byrne, in Istanbul, said more search and rescue teams were being sent from other parts of the country.
Hakki Erskoy, from the Turkish Red Crescent, said aid teams from the north and east of Turkey were being sent to the earthquake-hit area.
He said camps were being set up to shelter people and blankets, food and water were being sent along with mobile kitchens.
Military aircraft were being deployed to help with the rescue and relief efforts, Mr Erskoy told BBC World News.
However, Turkey has rejected all offers of foreign assistance, a foreign ministry official, who spoke on condition of anonymity, told Reuters.