BEIJING: A powerful earthquake struck a mountainous region of western China on Monday, reportedly killing more than 7,600 people, including as many as 5,000 people in a single county. The quake also trapped more than 900 students beneath a collapsed high school as tremors shook buildings throughout China and were felt as far away as Thailand and Vietnam, according to interviews and reports in China's state media. The 7.8 magnitude earthquake hit in Sichuan Province on Monday afternoon, and the death toll steadily increased throughout the evening, raising concerns that the number could go far higher. By 9 p.m. the state news agency Xinhua quoted provincial disaster relief officials as saying that 3,000 to 5,000 people were feared dead in Beichuan County. Roughly 80 percent of the buildings in the county were reportedly destroyed. An hour later, it reported 7,651 dead in Sichuan Province alone. Prime Minister Wen Jiabao, who arrived in the earthquake region Monday night, described the situation as a "severe disaster" and called for "calm, confidence, courage and efficient organization." President Hu Jintao ordered an "all out" effort to aid people in the earthquake region and soldiers were dispatched for disaster relief efforts. Minutes after the western temblor, a second, smaller quake struck hundreds of kilometers away, in an outer district of Beijing. Thousands of office workers were evacuated, but no damage was reported in the city, which is preparing to play host to the Olympics in August. "I suddenly felt very dizzy, as if I were heavily drunk," said Zeng Hui, who works on the 22nd floor of an office tower in Beijing. "I thought I was seriously ill, then I looked around and saw my colleagues felt the same way. We were stunned." The powerful initial quake struck at 2:28 p.m. near Wenchuan County, according to the State Seismological Bureau. The massive Three Gorges Dam, a few hundred kilometers east of the earthquake's epicenter, reported no immediate problems. "There were no signs that the earthquake has affected the dam and everything is going as usual," Hu Xinge, an executive with the state-owned corporation operating the dam, told Xinhua. Wenchuan is home to the Wolong Nature Reserve, the country's most famous panda reserve, and is located about 90 kilometers, or 55 miles, from Chengdu, the capital of Sichuan, which has a population of roughly 12 million people. Early reports and telephone interviews suggested that Chengdu had been spared any significant problems, but officials were struggling to assess the full scope of the damage in Wenchuan and elsewhere because of the disruption in communications caused by the earthquake. More than 2,300 cellphone towers were knocked down by the quake, said China Mobile, the country's top carrier. Damage is believed to be especially severe in Dujiangyan, a county of 600,000 people near the epicenter. One local official described rows of collapsed houses, Xinhua reported. Early Monday evening, Xinhua also flashed an emergency report from Dujiangyan saying that nearly 900 students were feared trapped after a high school collapsed. Most of the telephones in the city were not functioning and the report could not be independently verified. The first reports of fatalities came in the east in Chongqing Municipality, where two primary schools were damaged. Four pupils died and more than 100 were injured, state media reported. Another person was reportedly killed beneath a collapsed water tower in Sichuan Province. China is prone to seismic activity and has suffered horrific earthquakes in the recent past. In 1976, a 7.8 magnitude quake hit the city of Tangshan, roughly 110 kilometers from Beijing. More than 240,000 people were killed and nearly every building was leveled. Communist Party officials initially covered up the extent of the death toll. Many of China's biggest cities, including Beijing, are in high-risk earthquake zones. The smaller 3.9 magnitude earthquake Monday struck at 2:35 p.m. in Tongzhou, a district in the eastern half of Beijing. Many people in the city felt nothing at all, while others, especially those in high-rises, were alarmed by a swaying sensation. Thousands of workers were evacuated as a precaution. "Suddenly, everything around me started moving and swinging," said Xie Zhuofei, a salesman with a 17th floor office in Beijing. "I could hardly stand. Then I realized it was an earthquake. We went out immediately." Efforts to reach people near the epicenter of the bigger quake were hindered by the damaged telephone system. But receptionists at different hotels in Chengdu said the earthquake appeared not to have caused any major problems in the city. Xinhua showed photographs of minor flooding caused by damage to an underground water pipe, but the city seemed largely undamaged. The quake also disrupted air traffic control in western China, interfering with flights between Asia and Europe on Monday afternoon, although flight services were restored by the evening. Cathay Pacific Airways announced that it had canceled two flights between Hong Kong and London - one in each direction - and had delayed the departure of a Monday afternoon flight from Hong Kong to London by 19 hours, to Tuesday morning. "Cathay Pacific is monitoring the situation in Sichuan very closely," the company said in a statement on Monday evening. "Flights to and from Europe tonight are in normal operation." Keith Bradsher contributed reporting from Hong Kong and Zhang Jing contributed research.