Photographs posted on the internet, showing arms and a torso sticking out of the rubble of a school where an estimated 900 pupils were buried, gave some idea of the scale of the earthquake that has hit south-west China.

Desperate efforts were under way to find survivors in the collapsed ruins of Juyuan Middle School in Dujiangyan city, just south of the epicentre of the 7.8 magnitude quake in Sichuan province.

Scores of people were working frantically to free the trapped, using small winches or their hands to move concrete slabs.

China's state-run news agency Xinhua said 50 bodies had been pulled from the debris after the earthquake yesterday afternoon but did not say if they were alive.

It reported: "Some buried teenagers were struggling to break loose from underneath the ruins while others were crying out for help."

Gao Shangyuan, a local resident helping with the rescue effort, told the news agency he had run out of his house when the earthquake struck and saw some students escape before the building collapsed.

"Some had jumped out of the window and a few others ran down the stairs that did not collapse," he said.

Two girls said they managed to escape because they had "run faster than the others", the news agency reported.

"It was around 2.30pm and the building suddenly began to rock back and forth," one said.

Another photograph from Wenchuan, closest to the epicentre, showed what appeared to have been a six-storey building flattened, ripped away from taller buildings.

Casper Oppenhuisedejong, who works for a Dutch company in the Sichuan capital, Chengdu, near the epicentre, said: "All of a sudden I felt minor shocks and within seconds everybody was up. It was getting more and more intense. Everybody ran out.

"We were in quite a narrow street where everything just started shaking.

"All the alarms of the cars around went off, all the windows you heard smashing into each other.

"Entire buildings were being evacuated, people were panicking, especially since the phones didn't work. It was mayhem. Traffic got jammed. It was very surreal."

An employee of Sichuan's seismological bureau told Xinhua he had been driving near the epicentre when the earthquake struck. "The road started swaying as I was driving. Rocks fell from the mountains with dust darkening the sky over the valley."

The official death toll from the region of small cities and towns was put at around 8500 last night but with reports coming in all the time of collapsed hospitals, schools, factories and office blocks. it was expected to go much higher.

Chinese premier Wen Jiabao called the earthquake "a major geological disaster" and flew into Chengdu, which has a population of about 10 million, to oversee rescue and relief operations.

"Hang on a bit longer. The troops are rescuing you," Mr Wen shouted to people buried under the hospital of Dujiangyan city, on the road to Wenchuan.

"As long as there was a slightest hope, we should make our effort 100 times and we will never relax," he said outside the collapsed school.

The earthquake was one of the deadliest in the area for more than 30 years and compounded the problems of a government grappling with discontent over high inflation and an uprising among Tibetans in western China in the run up to the Olympic Games in Beijing in August.

Much of the affected area was closed to foreign media.

The earthquake lasted about three minutes and rattled buildings in the Chinese capital 930 miles north of the epicentre, causing office towers to be evacuated.

People ran screaming into the streets in other cities where many people said they had never felt an earthquake.

Willie McMartin, operational director with International Rescue Corps, a Scottish-based charity which supplies disaster rescue teams, said last night: "At the moment the Chinese authorities seem to have sufficient manpower to deal with such a disaster. Whether they have the technical knowledge that specialist groups like ours can provide remains to be seen.

"We are ready to go to the area at a moment's notice if our services are required."

David Booth, a seismologist with the British Geological Survey, in Edinburgh, said: "This is the biggest earthquake in south-west China for more than 30 years and is one of the biggest in the world this year. An earthquake in the area in 1933 killed about 9500 people.

"Although the Chinese are introducing earthquake resistant buildings, there remains a significant risk of high casualties because of the considerable increase in population and variations in geology which are not always taken into account in property design."