BEIJING: Beijing has moved to stem a tide of online criticism by tightening its grip on China's hugely popular microblogs, but experts say it will struggle to control the country's online masses.

China, which has the world's largest online population with 485 million users, constantly strives to exert its control over the Internet, blocking content it deems politically sensitive as part of a vast censorship system.

But the huge and rising popularity of weibos – microblogs similar to Twitter that have taken China by storm since they first launched two years ago – has posed a major challenge to the censors.

More and more Chinese people are turning to weibos to vent their anger over government corruption, scandals and disasters in a country where authorities maintain a tight grip on the media.

Though censors, many employed by the companies themselves, erase offending messages from the web as rapidly as they can, some stay online for hours or days before they are caught.

“This is where public opinion is being formed,” said Peking University journalism professor Hu Yong.

Hu said the decision by authorities in the booming east coast city of Dalian to relocate a controversial chemical plant owed much to a largely middle class public protest one Sunday in August that had its origins in weibo posts.

“The Dalian party secretary came out and gave a speech promising to shut the chemical plant,” he said. “We seldom see this. This is significant.”

Weibo users more than tripled in the first half of 2011, official data showed. Internet giant Sina.com said last month its weibo, by far the most popular, now has over 200 million users.

Weibo users can post commentary on others' messages, videos and images – including pictures of sensitive documents that might otherwise be censored – allowing information to spread rapidly in a country of 1.3 billion people.

A train crash that killed 40 people in July sparked an outpouring of public fury on the weibos, where thousands demanded to know why more care had not been taken over safety on China's flagship high-speed rail network.

The scale of the response appeared to take authorities by surprise. Shortly after the accident, the People's Daily, the mouthpiece of China's Communist Party, urged officials to use the weibos more to communicate with the public.

Weeks later, Beijing's most senior Communist Party official, Liu Qi, visited the offices of Sina and Youku, a Chinese site similar to YouTube, to urge them to stop the spread of “false and harmful information”.

Xiao Qiang, media scholar at the University of California at Berkeley, said the weibos made it easier for individuals to speak out, and harder for censors to pinpoint troublemakers.

“Weibo is a social media platform particularly effective at aggregating micro-opinions into a collective voice,” he told AFP.

“This mechanism of forming public opinion is new and effectively contesting the traditional method of control and censorship of the party.”David Bandurski of the China Media Project at the University of Hong Kong said attempts to censor the weibos were having an impact, with references to the mass protest in Dalian now removed.

“Censorship of overt references and images of the protests themselves is plainly dampening the social media impact,” he said.

But he said Beijing would not be able to “put the genie back in the bottle”, after web users' appetite for independently sourced information had been whetted.

China's leaders have made countless speeches in recent years urging the country's state-run media to become more open and less reliant on state subsidies, as they respond to the growing availability of information online.

“First China's leaders told the media to commercialise, which meant a drive to compete and professionalise. Now, weibo means the level of popular participation in the media is unprecedented,” Bandurski said.

The rise of microblogging has also forced changes in the way traditional state-run media operate.

Many newspapers were unusually critical of the government in the week that followed the July train accident – until Beijing's official propaganda department ordered them to stop.

And while authorities can still tell traditional media how to spin the news, Xiao said journalists were “increasingly putting otherwise censored materials online, on their blogs and then distributing them by weibo to the public”.

“The wisdom of the crowd will compete with the censors in a continuous battle that will play out over a long time,” predicted Peking University's Hu.