Students fought to escape from rubble and frantic rescuers dug out others buried in a school that collapsed following a Chinese earthquake that left thousands dead, state press reported Monday.

At least 50 people were confirmed killed and hundreds of others were buried alive in the collapse at the Juyuan Middle School in Sichuan province after a 7.8-magnitude earthquake struck southwest China, Xinhua news agency said.

Pictures posted on Chinese Internet news sites showed rescuers standing atop huge slabs of shattered concrete at the school as cranes tried to lift away massive chunks of rubble.

Altogether, 900 children were believed to be inside the school when the quake, with its epicentre about 100 kilometres (60 miles) away in Wenchuan County, struck.

Xinhua reporters on the scene said the three-story school building had partially collapsed. Some buried teenagers were struggling to break loose from underneath the ruins, while others were pinned under rubble and crying out for help.

Grieving parents watched as five cranes were excavating at the site as an ambulance waited.

One tearful mother told Xinhua that her son, Zhang Chengwei, was buried in the ruins.

Two girls, meanwhile, said they escaped because they had "run faster than others."

The quake struck in mid-afternoon, when Chinese schoolchildren were still in classes.

"It was around 2:30 pm (0630 GMT), and the building suddenly began to rock back and forth," one of them recalled.

A villager said the school had 18 classes, with about 50 students in each class.

"We ran out of the house when the quake hit," said Gao Shangyuan, a villager who lived close to the school and helped with the rescue work.

Gao and other villagers helped pull dozens of students out of the ruins.

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

China's Premier Wen Jiabao has described the earthquake as a major disaster that has left more than 8,500 people dead in Sichuan province alone, according to the latest tally. Casualties have also been reported in neighbouring provinces.