* Up to 5,000 dead in one county
* Eight schools toppled
* Rescuers struggling to reach worst-hit areas (Adds details, quotes)
By Lindsay Beck and Guo Shipeng
BEIJING, May 12 (Reuters) - A powerful earthquake in southwest China has killed up to 5,000 people and left as many as 10,000 injured, state media said, as hundreds of children remained trapped in at least eight collapsed schools.
An estimated 3,000 to 5,000 people were killed in Beichuan county of mountainous Sichuan province alone after the 7.8 magnitude earthquake hit the region during the early afternoon on Monday, Xinhua news agency said, citing the local government.
As many as 10,000 in Beichuan county were feared injured and 80 percent of the buildings there had been destroyed, the report said.
Beichuan's population is 161,000, meaning about one in 10 residents were killed or injured in the quake. The county is a part of Mianyang city, and about 160 km (100 miles) from the provincial capital, Chengdu.
The death toll was expected to rise sharply as authorities and rescue teams made contact with the worst-hit areas of Sichuan, where phone lines have been cut off since the quake struck.
It is now night in the affected area, hampering rescue efforts.
The quake had toppled at least eight schools and left hundreds of students and teachers trapped, state media said.
About 900 teenagers were buried in the rubble of a collapsed three-storey school building in the Sichuan city of Dujiangyan.
Local villagers had already helped dozens of students out of the ruins and five cranes were excavating at the site as anxious parents looked on, Xinhua said.
STUDENTS CRY OUT FOR HELP
"Some buried teenagers were struggling to break loose from underneath the ruins while others were crying out for help," the agency said.
Another seven schools had been felled by the quake, state media reported.
Five children were confirmed dead and 120 injured after buildings at two primary schools in rural parts of Chongqing municipality collapsed. Nineteen students and teachers were still buried, Xinhua said.
The U.S. Geological Survey said on its website (earthquake.usgs.gov) that the main quake struck at 0628 GMT at a depth of 10 km (6 miles).
At least 45 had died in the provincial capital, Chengdu, Xinhua said, citing an official with the local seismological bureau. Another 600 people were injured, 58 of them critically, in the sprawling city of 10 million.
State television showed footage of Chengdu residents, where the airport and railway station were closed, crowded in the streets looking relatively unscathed.
The quake's epicentre was in the nearby county of Wenchuan and its force caused buildings to sway across China and as far away as the Thai capital Bangkok.
Mountainous Wenchuan has a population of about 100,000 people.
Buildings were toppled in at least six counties near the epicentre, Xinhua said.
OLYMPIC STADIUM UNDAMAGED
In Beijing and Shanghai, office workers poured into the streets as the tremor hit. In the capital, which will host the summer Olympics in August, there was no visible damage and the showpiece Bird's Nest stadium was unscathed, the project's engineer told Xinhua.
But in Sichuan, phone lines in Wenchuan were down and a website for the region's Aba prefecture said the quake had cut several major highways and communications were down in 11 counties.
Premier Wen Jiabao arrived in Chengdu and President Hu Jintao ordered an "all-out" rescue effort, Xinhua reported.
Thousands of army troops and paramilitary People's Armed Police carrying medical supplies were also headed to the region, state television said. But a landslide had blocked a mountain road leading to Wenchuan, preventing troops from reaching the scene, state radio said.
Xinhua said there was no immediate impact to the Three Gorges Dam project, the weight of whose massive reservoir, hundreds of kilometres from Chengdu, experts have said could increase the risk of tremors.
A source at the biggest refinery in western China, Lanzhou, said the plant also appeared unaffected by the quake. (Additional reporting by Beijing and Shanghai bureaux and Darren Schuettler in Bangkok; Editing by Nick Macfie and David Fogarty)