he death toll quickly rose throughout the day. State-run news agency Xinhua said it had reached 8,533 in Sichuan Province by Monday night, and another 10,000 were believed to be injured.

It reported that authorities were yet to reach Wenchuan County -- which sits at the epicenter of the 7.9-magnitude earthquake with a population of about 112,000 -- because of damage to roads.

In Beichuan County, close to Wenchuan, the number of deaths was estimated at more than 3,000, with 80 percent of the buildings destroyed.  In addition, at least 48 people were killed in the northwest Gansu Province, Xinhua said. Several hundred students were also feared to be buried in collapsed school buildings, the agency said.

 Source : CNN World

China's Seismological Bureau said the earthquake had affected more than half the country's provinces and municipalities.

U.S. President George W. Bush released a statement saying his country "stands ready to help in any way possible."

"I am particularly saddened by the number of students and children affected by this tragedy," Bush said.

In Sichuan's Shifang city, the quake buried hundreds of people in two collapsed chemical plants, and more than 80 tons of ammonia leaked out, Xinhua said.

The local government evacuated 6,000 civilians from the area after homes and factories were also destroyed. 

The quake was "felt in most parts of China," Xinhua reported, with the confirmed casualties in the provinces and municipality of Sichuan, Gansu, Chongqing and Yunnan.

Xinhua said several schools collapsed, at least partially, in the quake.

At one, as many as 900 students were feared buried. At least 50 bodies have been pulled from the rubble at the high school in the Juyuan Township of Dujiangyan City in Wenchuan County.

"Some buried teenagers were struggling to break loose from underneath the ruins while others were crying out for help," Xinhua reported.

"Grieved parents watched as five cranes were excavating at the site and an ambulance was waiting.

"A tearful mother said her son, ninth-grader Zhang Chengwei, was buried in the ruins."

President Hu Jintao ordered an all-out effort to help those affected, and Premier Wen Jiabao traveled to the region to direct the rescue work, Xinhua reported.

"My fellow Chinese, facing such a severe disaster, we need calm, confidence, courage and efficient organization," Wen was quoted as saying.

"I believe we can certainly overcome the disaster with the public and the military working together under the leadership of the CPC Central Committee and the government."

Peter Sammonds, professor of geophysics at University College London, called the earthquake "tremendous." 

"Particularly in the more remote, the more mountainous part where this has taken place, a lot of the buildings are built on sediments that are quite unstable. They're probably liquifying, causing the buildings to collapse," he said.

"You might expect landslides to occur, which could actually stop the relief efforts going through on the roads, so this could be very grim in the remoter, more mountainous parts of this province."

While many of the most immediate efforts were focused on Sichuan Province, Xinhua reported that there were dead and injured also in Gansu, Chongqing and Yunnan.

Scattered stories of destruction poured in from around the country. Xinhua said one person was killed in Santai County, in the city of Mianyang, when a water tower fell.

A provincial government spokesman said they feared more dead and injured in collapsed houses in Dujiangyan City in Wenchuan County.

A driver for the seismological bureau said he saw "rows of houses collapsed" in Dujiangyan, Xinhua reported. 

"Wenchuan is home to the Wolong Nature Reserve, China's leading research and breeding base for endangered giant pandas," Xinhua reported. The condition of the center was not immediately known because all communication services were cut off.

Bonnie Thie, the country director the Peace Corps, was on a university campus in Chengdu about 100 km from the epicenter, in the eastern part of China's Sichuan province, when the first quake hit.

"You could see the ground shaking," Thie told CNN.

The shaking "went on for what seemed like a very long time," she said.

"This is a very dangerous earthquake," said Bruce Presgrave, a geophysicist with the U.S. Geological Survey, which updated the magnitude of Monday's quake from 7.8 to 7.9.

The quake had the potential to cause major damage because of its strength and proximity to major population centers, he said.

In addition, the earthquake was relatively shallow, Presgrave said, and those kinds of quakes tend to do more damage near the epicenter than deeper ones.

An earthquake with 7.5 magnitude in the northern Chinese city of Tangshan killed 255,000 people in 1976 -- the greatest death toll from an earthquake in the last four centuries and the second greatest in recorded history, according to the U.S. Geological Survey.

Tangshan is roughly 1,600 km from Chengdu, the nearest major city to the epicenter of Monday's quake. 

After the first quake struck Monday, the ground shook as far away as Beijing, which is 1,500 km from the epicenter.

They felt "a very quiet rolling sensation" that lasted for about a minute, according to CNN correspondent John Vause.

"Our building began to sway," he said.

Thousands of people were evacuated from Beijing high-rises immediately after the earthquake.

At least seven more earthquakes -- measuring between 4.0 and 6.0 magnitudes -- happened nearby over the three hours after the initial quake at at 2:28 p.m. local time (0728 GMT), the USGS reported.

A spokesman for the Beijing Olympic Committee said no Olympic venues were affected by the earthquake. The massive Three Gorges Dam -- roughly 600 km east of the epicenter -- was not damaged, a spokesman said.