Two chemical plants collapsed in Shifang city, to the southeast of the epicenter, burying hundreds of people and sending more than 80 tonnes of toxic liquid ammonia leaking, state media reported. At least one hospital was also flattened.

In Beichuan county, just east of the epicenter, 80 percent of the buildings had collapsed and some 10,000 people were injured on top of the 3,000 to 5,000 killed in the district, Xinhua News Agency said.

The situation in at least two other counties remains unclear.

The 7.8-magnitude earthquake, the most devastating in three decades, struck in the middle of the afternoon when classrooms and office towers were full 92 kilometers northwest of Sichuans capital Chengdu. Eight schools in the region completely or partially collapsed.

The quake emptied office buildings more than 1,500 kilometers away in Beijing; could be felt as far away as Vietnam; crashed telephone networks; and hours later, left parts of Chengdu, a city of 10 million, in darkness.

In Juyuan, Dujiangyan city, just south of the epicenter, at least 50 people were killed in one of the schools that partially collapsed, a three-story middle school.

Xinhua said its reporters saw buried teenagers at the site of the collapse struggling to break free from the rubble while others were crying out for help.

Photos posted on the internet showed arms and a torso sticking out of the rubble of the school as dozens of people worked to free the students, using small mechanical winches or their hands to move concrete slabs.

Another photo from Wenchuan, closest to the epicenter, showed what appeared to have been a six-story building flattened, ripped away from taller buildings of gray concrete. Xinhua reported students were also buried under five other toppled schools in Deyang city.

The government said late last night that thousands had died and that, besides those in Sichuan, the quake had caused deaths in three other provinces and Chongqing city. Beijing mobilized more than 5,000 soldiers and police to help rescue efforts in Sichuan and put the province on the second-highest level of emergency footing.

Premier Wen Jiabao, a geologist by training, called the quake a major geological disaster and flew into Chengdu to oversee the rescue and relief operations.

Stock markets in Shanghai and Shenzhen seesawed, dropping on inflation worries and then rising and tapering off over worries about the quakes economic impact to post slight gains.

The epicenter lies on a fault where South Asia pushes against the Eurasian land mass, smashing the Sichuan plain into mountains leading to the Tibetan highlands. Much of the area has been closed to foreign media and travelers, compounding the difficulties of getting information from the region.

Chengdus airport was closed. Power and telephone networks into the city and other affected areas were down, and panicked residents overloaded parts of the remaining telephone system with calls.

Residents fled into the streets and described an eerie feeling as people stayed outside into the night, fearing another quake. State media, citing the Sichuan seismology bureau, reported 313 aftershocks.

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

Although it was difficult to telephone Chengdu, an Israeli student, Ronen Medzini, sent a text message to the Associated Press saying there were power and water outages there.

"Traffic jams, no running water, power outs, everyone sitting in the streets, patients evacuated from hospitals sitting outside and waiting," Medzini said.

In Chongqing, Lai Dequn was napping while her mother watched TV on the 19th floor of a hotel. "I suddenly felt the bed shaking and then realized it must be an earthquake," said the 42-year-old Lai. "So I just put on slippers and helped my mother down to the ground floor."

State television broadcast tips for anyone trapped in the earthquake. "If you're buried, keep calm and conserve your energy. Seek water and food, and wait patiently for rescue," CCTV said.

The earthquake rattled buildings in Beijing 1,500 kilometers to the north. People ran screaming into the streets in other cities, where many residents said they had never felt an earthquake.

Some 1,100km to the east in Anhui province, chandeliers swayed in the lobby of the Buckingham Palace Hotel. "We've never felt anything like this our whole lives," said a hotel employee surnamed Zhu.

Patients at the Fuyang People's No1 Hospital were evacuated. An hour Leg 1after the quake, a half-dozen patients in blue-striped pajamas stood outside the hospital. One was lying on a hospital bed in the parking lot.

In Shanghai, skyscrapers swayed and office occupants went rushing into the streets.

In Beijing, where hundreds of thousands of visitors are expected for the Olympics, which start on August 8, venues for the Games were undamaged.

Li Jiulin, a top engineer on the 91,000-seat National Stadium - known as the Bird's Nest - was inspecting the venue when the quake occurred. He told reporters the building was designed to withstand an 8.0 quake.

"The Olympic venues were not affected by the earthquake," said Sun Weide, a spokesman for the Beijing organizing committee. "We considered earthquakes when building those venues."

Premier Wen, after arriving in Chengdu, traveled to Dujiangyan, near the collapsed middle school. He appealed for people to rally together.

"This is an especially challenging task," Wen said. "In the face of the disaster, what's most important is calmness, confidence, courage and powerful command."

The quake appears to be the deadliest since the most devastating in modern history, which killed 240,000 people in the city of Tangshan in 1976.

The Sichuan quake was centered about 10km below ground, the US Geological Survey said on its website. The depth of the quake made it so wide- ranging, seismologists said.

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