CHENGDU, CHINA - A powerful earthquake struck Western China on Monday, toppling thousands of homes, factories and offices, trapping students in schools, and killing at least 10,000 people, the country's worst natural disaster in three decades.

The quake, which was estimated to have a magnitude of 7.9, ravaged a mountainous region outside Chengdu, capital of Sichuan province.

It hit just after lunchtime Monday, destroying 80 percent of structures in some of the towns and small cities near its epicenter, Chinese officials said. Its tremors were felt as far away as Vietnam and set off another, smaller quake in the outskirts of Beijing, 900 miles away.

Landslides, power outages and fallen mobile phone towers left much of the affected area cut off from the outside world and limited information about the damage. But snapshots of concentrated devastation suggested that the death toll that could rise markedly as rescuers reach the most heavily damaged areas.

In Dujiangyan, south of the epicenter in the city of Wenchuan, a middle school collapsed, trapping 900 students in the rubble and setting off a frantic search for survivors that stretched through the night. Just east of the epicenter, a thousand students and teachers were missing or dead in a collapsed high school, The Associated Press reported.

Two chemical factories in Shifang were destroyed, spilling 80 tons of toxic, liquid ammonia, officials told Chinese state media.

The destruction of a steam turbine factory in the city of Mianzhu buried "several thousand" people, the Xinhua News Agency reported this morning.

The quake was China's biggest natural disaster since another earthquake leveled Tangshan in eastern China in 1976, killing 240,000 people and posing a severe challenge to the ruling Communist Party, which initially tried to cover up the disaster.

This time, officials mobilized some 50,000 soldiers to help with rescue efforts, state media said. Prime Minister Wen Jiabao flew to the scene and was shown coordinating disaster response teams from his jet.

The prime minister was later shown on national television standing outside the heavily damaged Traditional Medicine Hospital in the city of Dujiangyan, shouting encouragement at people trapped in its ruins. "Hang on a bit longer. The troops are rescuing you," he said. "As long as there is the slightest hope, we will never relax our efforts."

The powerful initial quake struck at 2:28 p.m. local time near Wenchuan County, according to China's State Seismological Bureau. Most of the heavy damage appeared to be concentrated in nearby towns, which by Chinese standards are not heavily populated. Chengdu, the largest city in the area with a population of about 10 million, is located about 60 miles away and did not appear to have suffered major damage.

But officials had yet to describe the impact in Wenchuan itself, which has a population of 112,000 and is home to the Wolong Nature Reserve, the largest panda reserve in China.

China's massive Three Gorges Dam, located a few hundred miles east of the earthquake's epicenter, reported no immediate problems.

At dawn today in Chengdu, clusters of people were huddled outside, many saying they were too fearful of aftershocks to go indoors. Many wore plastic slickers in the steady drizzle. Wang Zihong, 35, spent 12 hours outside his hotel, squatting with others on a street corner.

"It was a terrible shock," he said. "I couldn't stand up straight."