CHONGQING, China — A massive earthquake struck central China on Monday, killing more than 8,500 people, trapping nearly 900 students under the rubble of their school and spilling ammonia from a chemical plant, state media reported.

The 7.8-magnitude earthquake was among the worst to strike China in decades, devastating a hilly region of small cities and towns in Sichuan and nearby provinces. The official Xinhua News Agency said 8,533 people died in Sichuan and dozens of other deaths were reported elsewhere.

Xinhua said 80 percent of the buildings had collapsed in Beichuan county in Sichuan province after the quake, raising fears the overall death toll could increase sharply.

A chemical plant collapsed in Shifang city, to the northeast of the quake’s epicenter, burying hundreds of people and sending more than 80 tons of toxic liquid ammonia leaking from the site, state media reported.

The earthquake sent thousands of people rushing out of buildings and into the streets hundreds of miles away in Beijing and Shanghai. The temblor was felt as far away as Pakistan, Vietnam and Thailand.

The quake posed a challenge to a government already grappling with discontent over high inflation and a widespread uprising among Tibetans in western China while trying to prepare for the Beijing Olympics this August.

It hit about 60 miles northwest of Chengdu in the middle of the afternoon when classrooms and office towers were full. There were several smaller aftershocks, the U.S. Geological Survey said on its Web site.

The temblor struck hilly country leading up to the Tibetan highlands, toppling buildings in small cities and towns in the largely rural area. About 1,200 pandas — 80 percent of the surviving wild population in China — live in several mountainous areas of Sichuan.

The earthquake occurred in an area with numerous fault lines that have triggered destructive temblor before. A magnitude 7.5 earthquake in Diexi, Sichuan that hit on August 25, 1933 killed more than 9,300 people.

Xinhua said 50 bodies had been pulled from the debris of the school building in Juyuan town but did not say if the children were alive. Xinhua reported students also were buried under five other toppled schools in Deyang city.

Xinhua said its reporters saw buried teenagers struggling to break loose from underneath the rubble of the three-story building in Juyuan “while others were crying out for help.”

Two girls were quoted by Xinhua as saying they escaped because they had “run faster than others.”

Photos showed heavy cranes trying to remove rubble from the ruined school. Other photos posted on the Internet and found on the Chinese search engine Baidu showed arms and a torso sticking out of the rubble of the school as dozens of people worked to free them, using their hands to move concrete slabs.

Calls into the city did not go through as panicked residents quickly overloaded the telephone system. The quake affected telephone and power networks, and even state media appeared to have few details of the disaster.

“In Chengdu, mobile telecommunication convertors have experienced jams and thousands of servers were out of service,” said Sha Yuejia, deputy chief executive officer of China Mobile.