If Zhang Zhiyin had hesitated only a minute, he wouldn’t be alive today.

Now he wanders through the rainy streets of Dujiangyan, hoping to find friends who survived yesterday’s earthquake that levelled his school along with much of the city.

An estimated 900 teenagers lie buried under the rubble at the Juyuan Middle School.

“We had just finished lunch and were getting ready to go back to class. As soon as I left the building, everything started shaking,” Zhang told Reuters.

“I don’t know how many of my friends are alive. I’ve been wandering around town ever since. I don’t know where to go or what to do.”

Troops surround the school, keeping frantic relatives back from the rescuers working to free people from the rubble of the three-storey building.

“My child, my child!” cried one woman, grabbing at a soldier and pleading with him to be allowed past the security cordon.

A few of the most desperate try to push past the line of soldiers, desperate for news of their children. "We’re still pulling out people alive, but many, many have died," said one medical worker.

Survivors were being ferried to a nearby hospital, where a tarpaulin has been set up outside to treat victims. Other tarpaulins cover the body of the dead.

Hundreds, if not thousands, have been killed or are trapped under the rubble of what was once a sprawling city of more than half a million inhabitants. Today Dujiangyan was today a scene of devastation with buildings reduced to piles of broken concrete and bodies lying in the streets.

Troops and ambulances and military trucks block the roads, as soldiers race against time to clear rubble which has trapped an unknown number of people beneath it.

Wen Jiabao, the Chinese premier, and a geologist, flew to the town from Beijing last night and took his place at disaster relief headquarters in the city.

The country's Xinhua news agency reported that Mr Wen bowed three times in grief before some of the first 50 bodies pulled out from Zhang's school. "Not one minute can be wasted," said Mr Wen. "One minute, one second could mean a child’s life."

At a second school in the city, fewer than 100 of 420 students survived after a similar collapse, Xinhua reported.

Rescuers have worked frantically through the night, pulling bodies from homes, schools, factories and hospitals demolished by the quake, which rolled from Sichuan across much of China and was felt as far away as Bangkok and Hanoi.

Dujiangyan suffered some of the worst effects of China's most severe tremor in a generation.

Next to one wrecked apartment block, which once housed up to 70 people, a handful stood by the ruins cradling possessions in their arms. No-one can say how many are underneath.

"At least 60 or 70 old people lived there, as well as children," said a hospital worker surnamed Huo, gesturing to the building in ruins. Mattresses and household objects could be seen poking through the rubble. "How could they survive that?" she asked.

After the initial tremor yesterday, the torment was still not over for Dujiangyan residents. It was followed by a series of aftershocks, which shook the area through the night as rescuers tried to carry out their work. "Some are still very strong," a Dujiangyan resident told the Reuters news agency. "We have put up tents outside to sleep in."

The Sichuan quake was the worst to hit China since the 1976 Tangshan tremor in north-eastern China where up to 300,000 died.

As with so many urban centres in China, a country with a population of 1.3 billion, Dujiangyan is considered a small conurbation with its population of just 600,000. From now on however its name is likely always to be notorious because of the tragedy its people have suffered.

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