The official death toll in China's worst earthquake for three decades has risen to nearly 12,000 - and thousands remain missing.

Rescue workers continue to sift through the tangled debris of toppled schools and homes for thousands of victims buried or missing.

As night fell the day after the powerful 7.9 magnitude quake tore through urban areas and mountain villages across central China, rescue workers reached the epicentre in Wenchuan county, north of the Sichuan provincial capital, Chengdu.

Some 50,000 police and soldiers were mobilised for rescue efforts. The death toll was expected to jump sharply as rescuers worked their way through hard-hit towns at the epicentre.

Initial reports from soldiers who had to hike in over blocked roads showed there may be only 2,300 survivors from a population of 9,000 in Yinxiu.

People in the city of Mianyang, about 60 miles east of the epicentre, spent a second night sleeping outside in the rain, some under striped plastic sheeting strung between trees.

The government ordered people not to return to their homes, citing safety concerns, and posted security guards outside apartment complexes to keep people out. Few lights were on in the city of 700,000, and people ate and chatted by candlelight.

Rescue teams brought people evacuated from the hard-hit town of Beichuan to Mianyang's sports stadium for food and shelter.

In a massive government relief operation, some 20,000 soldiers and police arrived in the disaster area with 30,000 more on the way by plane, train, truck and on foot, the Defence Ministry said.

Rescue experts in orange jump-suits extricated bloody survivors on stretchers from demolished buildings.