China now has more than 500 million people on the Internet and nearly half use weibos, microblogs similar to Twitter that can circumvent the country's powerful censors, official data showed on Monday.

The weibos have become hugely popular platforms for people to vent their anger over perceived injustices or corruption and organise and spread news of protests, posing a challenge togovernment attempts to control information.

Almost 56 million Chinese -- equivalent to nearly the population of Britain -- used the Internet for the first time in 2011, taking the country's vast online population to 513 million.

China already had more Internet users than anywhere else in the world.

The rise in the number of people using the weibos was particularly dramatic, the China Internet Network Information Center (CNNIC) figures show, jumping to 250 million from just 63 million at the end of 2010.

"Chinese authorities are more and more concerned about the Internet because it's such a decentralised medium and so difficult to control," said David Bandurski, a Hong Kong-based researcher at the China Media Project.

"Since 2005, the whole focus of control of information has shifted from traditional media to the Internet."

China regularly blocks web content it deems politically sensitive in a vast censorship system dubbed the "Great Firewall of China" and is hugely concerned about the power of the Internet to influence public opinion.

But weibo users have been able to get around the controls by re-posting information and images as fast as the authorities can take them down.

A weibo user is believed to have broken the news of a deadly high-speed train crash in July that provoked widespread condemnation of the government -- much of it online -- and raised doubts about the expensive rail project.

News of a rare revolt against Communist officials in the southern village of Wukan last month first emerged on weibos and rapidly attracted worldwide media attention, before coming to a successful end after authorities pledged to investigate residents' grievances.

But in a bid to exert more control over microblogs, some cities such as Shanghai and Beijing now require weibo users to register under their real names, making it easier for authorities to track them.

Bandurski said some high-profile weibo users with huge followings had come under pressure recently as a result of a campaign to control the Internet.

"It is believed that they were issued with a number of warnings," he said.

CNNIC said that the number of people using more traditional communication tools such as emails, web forums or blogs was falling as the weibos grew in popularity.

Rural Internet use rose by 8.9 percent last year to 136 million people, but huge disparities still exist between rich and poor regions, the data shows.

While more than 70 percent of Beijing's population used the Internet last year, only 24.2 percent of people went online in the southwestern province of Guizhou -- the poorest in China.

The number of people surfing the web on mobile phones reached 356 million in 2011, up by nearly 53 million, the industry group said.

And whereas more than 30 percent of school children and students go online, only 0.7 percent of government and Communist Party cadres do, it added.

Overall, the group said that while the number of web users continued to rise in the world's most populous country, the rate of growth was gradually slowing.