Hard-hit parts of the county remained unreachable by road, with phone services cut off, but some text and Internet services continuing, state media said.
A person whose posts to a micro-blogging account "Qingyi Riverside" on Sina Corp.'s Twitter-like Weibo service carried a Lushan geotag said that many buildings collapsed buildings and that people could spot helicopters hovering above.
Aerial photos released by China's military and shown on state television showed individual houses in ruins and some stretches of the county seat and villages flattened into rubble. The roofs of some taller buildings appeared to have slipped off exposing the floors beneath them.
The official Xinhua News Agency reported that 56 people died. Xu Mengjia, Communist Party secretary for Ya'an, which administers Lushan, told CCTV that more than 600 people were injured.
The quake's shallow depth, less than 8 miles, likely magnified the impact and CCTV showed footages from local security cameras shaking. The official Xinhua News Agency said that the quake rattled buildings in the provincial capital of Chengdu 70 miles to the east. It caused the shutdown of the city's airport for about an hour before reopening, state media said.
Lushan, where the quake struck, is home to 1.5 million people where the fertile Sichuan plain meets foothills that eventually rise to the Tibetan plateau. The area is near a well-known preserve for pandas, Bifengxia, which Xinhua said was not affected by the quake. Dozens of pandas were moved to Bifengxia from another preserve, Wolong, after its habitat was wrecked by the 2008 quake.
Xinhua reported that more than 2,000 soldiers were being sent to the disaster area, along with specialized crews to restore telecommunications. Premier Li Keqiang flew to Ya'an to direct rescue efforts, and he and President Xi Jinping ordered officials and rescuers to make saving people the top priority, Xinhua said.
Social media users who said they were in Lushan county posted photos of collapsed buildings and reported that water and electricity had been cut off. At least 10 aftershocks some of them at magnitude-5 or higher shook the area.
"It's too dangerous," said a person with the Weibo account Chengduxinglin and with a Lushan geotag. "Even the aftershocks are scary."
The area lies near the same Longmenshan fault where the devastating 7.9-magnitude quake struck May 12, 2008, leaving more than 90,000 people dead or missing and presumed dead.
"It was just like May 12," said Liu Xi, a writer in Ya'an city, who was jolted awake by Saturday's quake. "All the home decorations fell at once, and the old house cracked."