Rescue workers sifted through tangled debris of toppled schools and homes Tuesday for nearly 19,000 victims buried or missing after China's worst earthquake in three decades, where the death toll soared to more than 12,000 people in the hardest-hit province alone, state media reported.

Hope that many survivors would be found was fleeting. Only 58 people were extricated from demolished buildings across the quake area so far, China Seismological Bureau spokesman Zhang Hongwei told the official Xinhua News Agency. In one county, 80 percent of the buildings had been destroyed.

"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.

A day after the powerful 7.9 magnitude quake struck Monday afternoon, state media said rescue workers had reached the epicenter in Wenchuan county, but the number of casualties there was still unknown. The quake was centered just north of the Sichuan provincial capital Chengdu in central China, tearing into urban areas and mountain villages.

• Click here for photos.

Click for additional earthquake information and shake maps from the USGS.

Click here for security camera footage.

Rain was impeding efforts, and a group of paratroopers called off a rescue mission to the epicenter due to heavy storms, Xinhua reported.

The official death toll climbed past 12,000 in Sichuan province alone, but difficulties in accessing some areas meant the total number of casualties remained uncertain.

The number of victims was expected to rise, with 18,645 still buried in just the city of Mianyang near the epicenter, Xinhua reported. People there 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.

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

Some 20,000 soldiers and police arrived in the disaster area with 30,000 more on the way by plane, train, truck and even on foot, the Defense Ministry told Xinhua. Rescue experts in orange jumpsuits extricated bloody survivors on stretchers from demolished buildings.

Aftershocks rattled the region for a second day, sending people running into the streets in Chengdu.

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

"My wife died in the quake. My house was destroyed," he said. "I am going to Chengdu, but 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 epicenter, 1,000 students and teachers were killed or missing at a collapsed high school in Beichuan county — a more than six-story building reduced to a pile of rubble about two yards (meters) high, according to Xinhua. The agency said up to 5,000 people were killed and 80 percent of the buildings had collapsed in Beichuan alone.

At another leveled 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.

"We're still pulling out people alive, but many, many have died," one medical worker told Reuters.

Click here to read more about the trapped students.

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

The Ministry of Health issued an appeal for blood donations to help the quake victims.

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 center 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.

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

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 epicenter 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. Russia was sending a plane with rescuers and supplies, the country's Interfax news agency reported.

But Wang, the disaster relief official, said international aid workers would not be allowed to travel to the affected area. While China has been slow at times to react to natural disasters, it usually has not lacked the manpower to eventually overcome the problem.

China's Ministry of Finance said it had allocated $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. Financial analysts said the quake would have only a limited impact on the country's booming economy.

The Associated Press contributed to this report.