New Delhi, May 12 : Between 3,000 and 5,000 people are feared to have been killed by an earthquake measuring 7.8 in just one county of south-western China's Sichuan province, reports say.
Some 10,000 people are also feared to have been injured in Beichuan county.
The official Xinhua news agency said that nearly 900 students had been buried in the densely populated areas of southwestern China.
At least four children were killed and over 100 were injured when the quake toppled two primary schools in the city of Chongqing near the epicentre.
Another person was killed in neighbouring Sichuan province by a collapsing water tower.
The quake struck 93 kilometres from Chengdu, the capital of Sichuan province, and about 260 kilometres from Chongqing.
The State Seismological Bureau located its epicentre in Wenchuan County. Buildings shook in Beijing and Shanghai, residents reported, however there were no immediate reports of damage there. Tremors were also felt in Bangkok, Hong Kong, Hanoi and Taipei, residents said.
China's Three Gorges Dam was not damaged by the quake, Xinhua said in a separate report.
Details of their plight were still coming through but reports said that the students were buried when their school building collapsed in Dujiangyan.
Buildings also collapsed in the town of Wenchuan. Communications to the town appeared to have been severed, with calls failing to connect to both fixed line and mobile telephones.
President Hu Jintao has urged "all-out" efforts to rescue victims of the quake. Chinese Premier Wen Jiabao is trying to reach the epicentre after the tremor rocked the region at a depth of 18 miles this afternoon.
The decision to send Jiabao was the first hint that officials feared a high casualty toll in a region with a large Tibetan population that has seen anti-Chinese unrest in recent weeks.
Troops have already been sent to help with disaster relief work in Wenchuan, a city of about 111,000 people in Sichuan.
Wenchuan is best known as the home of the Wolong Nature Reserve, China's leading research and breeding base for the endangered giant panda. Calls to the base failed to connect.
The government in Aba prefecture of Sichuan province said buildings had collapsed and many were cracked. Mountain roads had been damaged.
The earthquake was felt as far away as Vietnam and Thailand, startling office workers in high-rise buildings. There were no reports of damage. In Beijing, office workers rushed in panic out of towers in the capital's financial district.
One of China's tallest buildings, the Jinmao Tower in Shanghai, as well as other skyscrapers were ordered to evacuate after the tremor and aftershocks.
There are harrowing reports from the scene of the school collapse in Dujiangyan city - about 100km (60 miles) from the epicentre.
Teenagers buried beneath the rubble of the three-storey Juyuan Middle School building were struggling to break free, while others were crying out for help, state news agency Xinhua reported.
Parents were watching as cranes excavated the site. Villagers rushed to help with the rescue.
Two girls said they escaped because they had "run faster than others".
Dozens of aftershocks have been reported since the quake, which was the strongest to hit Sichuan province in more than 30 years, Xinhua reports.
State television said the quake had not caused major damage to Chengdu, which has a population of more than 10 million people.
China's worst earthquake in modern history was centred in the northern city of Tangshan and killed some 250,000 people on July 27, 1976. That tremor lasted for 15 seconds and flattened 90 percent of buildings. It was the deadliest in the world of the 20th century.