MIANYANG, China - Rescue workers sifted through tangled debris of toppled schools and homes Tuesday for thousands of victims buried or missing after China's worst earthquake in three decades, as the death toll soared to more than 12,000 people in the hardest-hit province alone.

As night fell the day after the powerful 7.9 magnitude quake tore through urban areas and mountain villages, rescue workers reached the epicentre in Wenchuan county, north of the Sichuan provincial capital, Chengdu.

Some 50,000 police and soldiers were mobilized for rescue efforts. The death toll was 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 2,300 survivors from a population of 9,000 in Yinxiu, one of the affected towns, state TV quoted local emergency official He Biao as saying.

People in the city of Mianyang, about 100 kilometre east of the epicentre, spent a second night sleeping outside in the rain, some under striped plastic sheeting strung between trees. 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 the city of 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."

Rescue teams brought people evacuated from the hard-hit town of Beichuan to Mianyang's sports stadium for food and shelter. Outside the railway station, police shouted through megaphones telling people where they could get free rice porridge.

Buses carrying survivors headed away from Beichuan, which was flattened by the quake. Footage on CCTV showed few buildings standing amid piles of rubble in a narrow valley. The six-storey Beichuan Hotel sat listing to one side, half its first floor collapsed. Medical teams tried to treat the injured in dirt courtyards littered with broken furniture and rubble.

In a massive government relief operation, some 20,000 soldiers and police arrived in the disaster area with 30,000 more on the way by plane, train, truck and on foot, the Defence Ministry told the official Xinhua news agency. Rescue experts in orange jumpsuits extricated bloody survivors on stretchers 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, told reporters in Beijing.

Xinhua said more than 12,000 had died in Sichuan province alone, but difficulties in accessing some areas meant the total number of casualties remained uncertain. In counties around one city near the epicentre, 18,645 people remained buried, the agency said.

More than two dozen British and American tourists who were thought to be panda-watching in the area also remained missing.

Zhou Chun, a 70-year-old retired mechanic, was leaving quake-hit Dujiangyan city with a soiled, light-blue blanket draped over his shoulders.

"My wife died in the quake. My house was destroyed," he said. "I don't know where I'll live."

Zhou and other survivors were pulling luggage and clutching plastic bags of food amid a steady drizzle and the constant wail of ambulances.

Just east of the epicentre, 1,000 students and teachers were killed or missing at a collapsed high school in Beichuan county - a more than six-storey building reduced to a pile of rubble about two metre high, according to Xinhua.

At another levelled school in Dujiangyan, 900 students were feared dead. As bodies of teenagers were carried out on doors used as makeshift stretchers, relatives lit incense and candles and also set off fireworks to ward away evil spirits.

Elsewhere in Gansu province, a 40-car freight train derailed in the quake that included 13 gasoline tankers was still burning Tuesday evening, Xinhua said.

Premier Wen Jiabao, who rushed to the area to oversee rescue efforts, said a push was on to clear roads and restore electricity as soon as possible.

His visit to the disaster scene was prominently featured on state TV, a gesture meant to reassure people that the Communist party was doing all it could.

"We will save the people," Wen said through a bullhorn to survivors as he toured the disaster scene, in footage shown on CCTV. "As long as the people are there, factories can be built into even better ones, and so can the towns and counties."

Fifteen missing British tourists were believed in the area at the time of the quake and were "out of reach," Xinhua reported.

They were likely visiting the Wolong Nature Reserve, home to more than 100 giant pandas, whose fate also was not known, Xinhua said, adding that 60 pandas at another breeding centre in Chengdu were safe.

Another group of 12 Americans also on panda-watching tour sponsored by the U.S. office of the World Wildlife Fund remained out of contact Tuesday, said Tan Rui, WWF communications officer in China.

Two Chinese-Americans and a Thai tourist also were missing in Sichuan province, the agency said, citing tourism officials.

The disaster comes less than three months before the start of the Beijing Olympics. The tragedy is just the latest event to tarnish the run-up to the event meant to showcase China's rise that has been marked by internal strife and criticism abroad of Beijing's human rights record.

In light of the quake, Beijing Olympics organizers said the torch relay will be simplified, downscaled and begin with a minute of silence Wednesday when a leg kicks off in the southeastern city of Ruijin.

The more somber relay will likely last until the torch's previously planned trip to the quake-hit areas next month, organizing committee spokesman Sun Wiede said, and people along the route will be asked for donations to help disaster victims.

The Dalai Lama, the exiled spiritual leader who has been vilified by Chinese authorities who blame him for recent unrest in Tibet, offered prayers for the victims. The epicentre is just south of some Tibetan mountain areas that saw anti-government protests earlier this year.

The Chinese government said it would welcome outside aid supplies, but not relief workers. Still, Russia was sending a plane with rescuers and supplies, the country's Interfax news agency reported.

China's Ministry of Finance said it had allocated the equivalent of US$123 million in aid for quake-hit areas.

The quake was China's deadliest since 1976, when 240,000 people were killed in the city of Tangshan, near Beijing in 1976.

Seismologists said the quake was on a level the region sees once every 50 to 100 years. The last time the Indian land mass crashed the Tibetan highlands into the Sichuan plane with such ferocity was 1933, when a magnitude 7.5 quake killed more than 9,300 people. Monday's quake resulted from the pent-up stress since then, experts said.

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