BEIJING (Reuters) - The heaviest rain storm in six decades to hit the Chinese capital killed at least 10 people and caused widespread chaos, flooding streets and stranding 80,000 people at the city's main airport, state media reported on Sunday.

The storm, which started on Saturday afternoon and continued late into the night, flooded major roads and sent torrents of water tumbling down steps into underpasses.

In the Beijing suburb of Tongzhou, two people died in a roof collapse and another person killed was struck by lightning, the official Xinhua news agency reported.

Other deaths were caused by electric shocks from downed power lines and drowning, it added, without giving an exact breakdown.

More than 500 flights were cancelled at Beijing's Capital International Airport, the Beijing News said.

However, the subway system was largely unaffected, aside from being swamped with people desperate to get home and unable to use cars, buses or taxis.

The city received about 170 millimeters (6.7 inches) of rain on average, though a township in Fangshan District to Beijing's west was hit by 460mm (18.1 inches), Xinhua said.

The Beijing city government said on its website (www.beijing.gov.cn) it was working to get the metropolis back on its feet, but reminded people to prepare for further bad weather.

"The weather forecasters say that from late July to early September this city is prone to flooding, and there could be further large-scale storms or extreme weather," it said.

Many residents took to China's popular micro blogging site Sina Weibo to post dramatic pictures of the storm. Some complained the city should have been prepared, especially as the government had issued a severe storm warning the day before.

"It was forecast early on that Beijing would get torrential rain, so why were pumps and other facilities not prepared in time?" complained one user.

But at least one good result came from the storm.

The official pollution index, which had showed an unhealthy rating before the storm hit, registered "excellent" on Sunday, with the air noticeably free of its normal acrid smell.

(Reporting by Ben Blanchard; Editing by Ed Lane)