Bodies covered with sheets lined streets yesterday as rescue workers dug through schools and homes turned into rubble by China's worst earthquake in more than three decades to rescue victims trapped under concrete slabs.

Rescuers sifted through the tangled rubble as the death toll jumped to more than 12,000 people in the hardest-hit province alone.

As darkness fell, a day after the powerful 7.9 magnitude quake tore through urban areas and mountain villages across south-west China, rescue workers reached the epicentre in Wenchuan county, north of the Sichuan provincial capital, Chengdu.

About 50,000 police and soldiers were mobilised for rescue efforts. The death toll was again expected to jump sharply as rescuers worked their way through hard-hit towns at the epicentre.

Initial reports from soldiers who had to hike in over blocked roads showed there may be only 2300 survivors from a population of 9000 in Yinxiu, one of the affected towns.

In the city of Mianyang, about 60 miles east of the epicentre, more than 18,000 people are said to be buried under the rubble, the Xinhua news agency reported.

It said at least 4800 people were also trapped in the nearby town of Mianzhu and massive landslides had buried roads to outlying villages.

The government ordered people not to return to their homes, citing safety concerns, and posted security guards outside apartment complexes to keep people out.

Few lights were on in Mianyang, which has a population of about 700,000, and people ate and chatted by candlelight.

"My heart was so uneasy last night, I couldn't sleep," said Wen Dajian, wrapped in a floral quilt lying on the rickshaw he uses to make a living hauling goods. "I'm still so scared tonight. There's no place for me to go."

In a massive government relief operation, about 20,000 soldiers and police arrived in the disaster area with 30,000 more on the way by plane, train, truck and on foot, said defence ministry officials. Rescue experts in orange jumpsuits extricated bloodstained survivors by stretcher from demolished buildings.

"Survivors can hold on for some time. Now it's not time to give up," Wang Zhenyao, disaster relief division director at the Ministry of Civil Affairs, said in Beijing.

However, attempts to find survivors have become a race against time and bad weather. Premier Wen Jiabao, who has thrown himself into the task of co-ordinating these efforts, sounded a grim note.

"The disaster situation is worse than expected, and the rescue sites are quite complex," Wen said. He ordered troops to clear roads to Wenchuan. "Please speed up the shipping of food. The kids have nothing to eat now," he said amid crying children.

Just east of the epicentre, 1000 students and teachers were killed or missing at a collapsed high school in Beichuan county. The six-storey building was reduced to a pile of rubble about 6ft high, according to Xinhua.

At another levelled school in Dujiangyan, 900 pupils were feared dead.

As bodies of teenagers were carried out on doors used as makeshift stretchers, relatives lit incense and candles and set off fireworks to ward off evil spirits.

Elsewhere in Gansu province, a freight train derailed by the quake, whose 40 cars included 13 petrol tankers, was reported to be still burning last night.

Organisers of the Beijing Olympics, to be held in August, said the torch relay would be scaled down and will begin with a minute's silence today when a leg kicks off in the south-eastern city of Ruijin.

Expressions of sympathy and offers of help have poured in from across the world. President Hu Jintao discussed the disaster in a phone call with George W Bush, Chinese TV reported.

The Chinese government said it would welcome outside aid supplies, but not relief workers. It has allocated more than £60m in aid for areas hit by the earthquake which was China's worst since 1976, when 240,000 people were killed in the city of Tangshan, near Beijing.