Seven people died and 88 were injured in the quake late on Tuesday, 21 of them seriously, the official Xinhua news agency said.
It said all the dead were visitors to the area.
But China's national commission for disaster reduction estimated that as many as 100 people may have died, based on 2010 census data of the mountainous, sparsely populated region.
More than 130,000 houses may be damaged, it added in a statement posted on its website, based on a preliminary analysis of the disaster in a remote region of Sichuan province.
The president, Xi Jinping, called for "all-out efforts to rapidly organise relief work and rescue the injured people", according to Xinhua.
Tang Sesheng, a restaurant owner, fled her establishment in the town of Jiuzhaigou after she felt the earth moving under her.
"I was also in Jiuzhaigou in 2008 during the last big quake, so I knew what it was.
This felt even stronger," she told AFP.
She said people had come out of their homes to sit out in the town's large public square, far from any tall structures, afraid to go back inside for fear that buildings might topple.
Many were also sitting in cars, thinking it safer.
She added: "People didn't dare grab anything like money or clothes - we just all ran outside right away."
The quake occurred about 9.20pm (1320 GMT), not far from the site of a massive magnitude 8.0 earthquake that struck in 2008 leaving 87,000 people dead or missing.
Its epicentre was 284 kilometres (176 miles) north of the provincial capital Chengdu and struck at a depth of 10 kilometres, the US Geological Survey said.
The affected region, Jiuzhaigou county, includes one of the country's most famous national parks, a Unesco World Heritage Site known for karst formations and lakes.
More than 38,000 people visited the tourist site on Tuesday, according to the official Xinhua news agency, which added that electricity was cut off briefly.
Some houses at the scenic spot collapsed after the quake and authorities were organising young people to help evacuate residents, a staff worker at the park told Xinhua.
Photos from the area posted on social media showed masses of people milling about on streets scattered with light debris, and a taxi dented from being hit by a large boulder.
More than 600 fire officers and soldiers have been deployed, the People's Daily newspaper said.
The Red Cross Society of China said it was deploying emergency specialists and volunteers to assist affected communities.
"The quake hit at night, communications lines and electricity are disrupted and people are no doubt shocked and scared," said Gwendolyn Pang, a spokeswoman for the International Federation of Red Cross and Red Crescent Societies in China.
"It may take some time to learn the extent of damage and casualties."
China is regularly hit by earthquakes, especially in its mountainous western and south-western regions.
Earlier on Tuesday, a landslide triggered by heavy rains killed at least 24 people in a different, mountainous region of Sichuan to the south of Chengdu.