Smash hit

President Bush was quick to offer humanitarian help. "The thoughts and prayers of the American people are with the Chinese people, especially those directly affected. The United States stands ready to help in any way possible," Mr Bush said in a statement.

The People's Liberation Army was ordered to help with the disaster relief work. The international airport at Chengdu, the capital of Sichuan province, was closed and air travel disrupted elsewhere.

An Olympic spokesman said that none of the 31 venues for the Beijing Olympics in the capital and six other host cities had been damaged. "They are earthquake-proof to a high degree and no damage was done," said Sun Weide of the Olympic communications department.

Both the Chinese seismological bureau and the US Geological Survey, which use different scales, measured it at 7.8. A series of aftershocks continued to rock the region, including one of 5.8 felt near Chengdu, according to the USGS.

The phone network there and elsewhere around the country appeared to suffer a meltdown as people tried to find out what happened.

Resident of Chengdu said that they felt a violent shaking that threw glassware to the floor and toppled street lights.

Troops were immediately sent to help with disaster relief work in Wenchuan, a city of about 111,000 people.

Tian Yixiang, who works for the emergency office of the People’s Liberation Army, said the soldiers would assist the local government there. 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.

A resident of Mianyang, in Sichuan, said: "The air-conditioning unit fell off the wall. Vases are all broken. The sick in hospital have been moved outside to open fields. There is no electricity and no mobile phone reception. People are afraid of aftershocks."

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 per cent of buildings. It was the deadliest in the world of the 20th century.

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