The Xinhua news agency said at least four school children had died and many had been injured as two primary schools collapsed in the Chongqing municipality.
The precise death toll has yet to be established due to the widespread destruction of local infrastructure, including communications.
Chinese national TV reported that further tremors were expected late on Monday, including in Beijing.
The U.S. Geological Survey said on its website that the quake struck at 2:28 p.m. local time (06:28 GMT), and that its epicenter was 57 miles (92 km) northwest of Sichuan's provincial capital, Chengdu.
Tremors were also felt some 930 miles (1,500 km) away in the capital, Beijing, where office workers rushed out into the streets as their buildings shook.
According to press reports, tremors could also be felt as far away as Bangkok, the Thai capital, and Hanoi, the capital of Vietnam.
Xinhua also said that President Hu Jintao had sent Premier Wen Jiabao to the area to coordinate relief work. The military is also reported to have been sent to the area.
"I am afraid we could have a repeat of 1976, when not far from Beijing an earthquake claimed thousands of lives," a resident of the Chinese capital told RIA Novosti, referring to the most deadly earthquake of the 20th century, which destroyed the city of Tangshan, killing at least 240,000 people and badly injuring over 160,000.
