Turkey earthquake Hundreds Feared Dead
Dozens of buildings were destroyed by the blast in the city of Van, trapping some victims alive under debris.
At least 75 people were believed to have been killed, although officials said the final toll could reach 1,000.
Emergency workers and residents were battling to dig people out of the rubble, using their bare hands.
Halil Celik, standing beside a ruined building, said: "We heard cries and groaning from underneath the debris, we are waiting for the rescue teams to arrive."All of a sudden, a quake tore down the building in front of me. All the bystanders, we all ran to the building and rescued two injured people from the ruins."
At another site, three teenagers were believed to be trapped under a collapsed building.
People clambered over the shattered masonry, shouting: "Is there anyone there?"
A dozen buildings collapsed in Van, which is close to the Iranian border, and more were brought to the ground in the nearby district of Ercis.
Turkey's state-run television also said a group of inmates have escaped from a prison in the area.
Turkey's Kandilli Observatory and Earthquake Research Institute said the earthquake was 7.2 in magnitude.
It had a depth of 20km (12.4 miles), which is relatively shallow and could potentially cause more damage.
Mustafa Erdik from the Kandilli Observatory said: "We estimate around 1,000 buildings are damaged and our estimate is for hundreds of lives lost. It could be 500 or 1,000."
Deputy Prime Minister Besir Atalay said: "Around 10 buildings have collapsed in the city of Van and around 25 or 30 have collapsed in Ercis, including a dormitory."
Hospital sources in Ercis, a town near the quake's epicentre, said there were more than 50 dead bodies at one hospital and that 405 people had been wounded.
Foreign Secretary William Hague said: "The earthquake in eastern turkey has caused immense destruction, resulting in terrible loss of life.
"I send my deepest condolences to the friends and loved ones of those who have died or been affected.
"My thoughts are with those who are anxiously waiting news from the rescue efforts.
"The UK is ready to provide assistance, in line with any request from the Turkish authorities. We stand with Turkey at this difficult time."
Earthquakes are not rare occurences in Turkey, many parts of which lie on a major seismic fault line.
In 1999, about 18,000 people were killed by two powerful earthquakes that struck Izamit in the north west.