A powerful earthquake has killed at least 160 people and injured at least 5,700 in China's rural south-west, officials say.
The 6.6-magnitude tremor sent people fleeing from buildings across Sichuan province, which was devastated by a massive quake five years ago.
Villages close to the epicentre in Lushan county were left in ruins.
The rescue operation is being hampered by collapsed roads, broken telephone lines and regular aftershocks
Thousands of troops have been sent to Sichuan, and Premier Li Keqiang has arrived in the area.
"The current most urgent issue is grasping the first 24 hours since the quake's occurrence, the golden time for saving lives," Mr Li was quoted as saying by Xinhua state news agency.
Rescuers have been able to pull some bodies and survivors from the rubble of devastated villages.
There have been at least 710 aftershocks, further damaging buildings and leaving them dangerous.
Power and water supplies have been knocked out in Lushan county.
The quake struck at 08:02 local time (00:02 GMT) on Saturday, with the China Earthquake Administration categorising it as a 7.0 magnitude, and the US Geological Survey (USGS) reporting it as 6.6.
Its epicentre was 115km (70 miles) west of provincial capital Chengdu, according to the USGS.
A square outside the Lushan county hospital has been turned into a triage centre, with dozens of people being treated in tents.
State broadcaster CCTV showed images of bloodied people being treated in tents.
One injured man told the channel: "We still live in our old house, the new one is not ready yet. Our house just collapsed. Everything collapsed."
The quake was measured at 12km below the surface - a shallow depth that usually indicates extensive damage.
CCTV footage suggested entire villages around the epicentre had been flattened.
People in Chengdu felt the tremor and came running into the streets wrapped in blankets.
Chengdu resident Aaron Ozment told the BBC there was huge confusion in the city.
"I threw on a some clothes quickly and made my way into the courtyard of my complex," he said.
"Making calls was almost impossible; everybody was trying to contact everybody they knew."
Residents in the nearest city to the epicentre, Ya'an, felt jolts from the quake and aftershocks, but the city does not appear to have suffered major damage.
Correspondents said almost 30,000 rescuers had begun work in the quake zone. The Chinese premier was flown to Ya'an by helicopter as soon as he arrived in Sichuan, Xinhua reported.
Aircraft had begun flying over the area to assess the damage and deliver supplies.
Five years ago a massive quake hit Sichuan, killing tens of thousands.
The 2008 disaster left some five million people homeless.
Many of the collapsed buildings were schools and nurseries, leading to widespread criticism of local government's planning policies.