China has repeatedly criticized microblogs for spreading what it calls unfounded rumors and vulgarities and has issued a series of warnings that online content must be acceptable to the ruling Communist Party.

Microblogs such as Sina's Weibo allow users to issue short messages of opinion -- a maximum of 140 Chinese characters -- that can course through chains of followers who receive messages instantly.

Censors have a hard time monitoring the tens of millions of messages sent every day and users have become expert at using clever, nuanced language to discuss sensitive topics such as human rights and the foibles of the top leadership.

Now, in rules unveiled by the Chinese capital's government and carried by state media, individual and company users must register with their real identification information.

Users have three months to register with "responsible departments for Internet content" or will face legal consequences, state media cited the rules as saying.

However, people will be able to choose their own user names, state-run Xinhua news agency cited an unidentified government spokesman with the Beijing Internet Information Office as saying.

Hong Kong media said the cities of Shanghai and Guangzhou were likely to follow suit.

"Not only will this not affect the development of microblogs, it will help such sites build their brands and improve their service," the government spokesman told Xinhua.

China has more than 300 million registered microbloggers, although many people have more than one account.

Wang Junxiu, an Internet commentator and investor in Beijing, said the new policy would be difficult to implement -- the rules give no details on how they will be enforced -- but nonetheless would have a chilling effect.

"I don't know how they're going to implement this because there are already hundreds of millions of users on microblogs," said Wang, who studies microblog developments.

"How do you go about checking them one by one? It will be very hard to enforce, but it still means that the intensity of controls will grow."

Peng Shaobin, general manager of Sina's microblog service department, told Xinhua that the company had been trying hard to "stop the spread of false information" on microblogs.

"We ... support the regulations," Peng said.

China already blocks foreign social media sites like Twitter and Facebook, fearing the uncensored sharing of images and information could cause instability and harm national security.

DISMAY ONLINE

Chinese microbloggers were quick to share their dismay at the new rules.

"This is a covert way of monitoring what people say and to control public opinion!" complained a user with the screen name of "huitailang." "But think about it another way -- if the subjects of heaven are full of complaints, that's bad for social harmony and for our emperors."

Still, investors may be slightly more relaxed, as they have been expecting tighter regulation for some time now, said Dick Wei, a Hong Kong-based analyst with JPMorgan.

The impact would depend on how the new regulations were executed, he added.

"If you look at the real-name registration for the online-games industry back a couple of years ago, the execution of the real-name registration on the online games industry did not impact the online game usage," Wei said.

Sina and other Chinese microblog operators already deploy technicians and software to monitor content and block and remove comments deemed unacceptable, especially about protests, official scandals and Party leaders.

While the Party has vowed to intensify control over online social media and instant messaging tools, analysts say it is unlikely to shut down what has become an important valve for monitoring and easing social pressures.

The government has also urged ministries, officials and the police to set up their own microblogging accounts to give the public "correct" facts and release authorized information to dispel misunderstandings.

(Additional reporting by Chris Buckley and Huang Yan in BEIJING and Melanie Lee in SHANGHAI)

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