Associated Press
In this photo taken Wednesday, July 14, 2010, a Chinese man uses a computer at an Internet cafe in Beijing, China.

BEIJING — Beijing authorities on Friday ordered Internet microblogs to require users to register with their real names, a tightening of rules aimed at controlling China's rapidly growing social networks.

An announcement posted online said all microblog companies registered in the capital had to enforce real name registration within three months.

The rules, jointly issued by the Beijing government, police and Internet management office, apparently apply to all 250 million users of the hugely popular Twitter-like service, regardless of location, because its operator, Chinese Web portal Sina Corp., is headquartered in Beijing.

Sina rival Tencent Holdings is based in the southern city of Shenzhen. It wasn't immediately clear whether the company's microblog service would have to comply with the same rules.

China had more than 485 million Internet users as of the end of June, the most of any country in the world.

Government officials warned in October that tighter new guidelines for social media sites were coming. Officials said then they were concerned about people using the Internet to spread lies and rumors. But the government is also clearly worried about the use of Weibo and other sites to mobilize potentially destabilizing protest movements.

The new rules explicitly forbid use of microblogging to "incite illegal assembly." Public protests are illegal in China and are a concern for the Communist leadership.

Microblogs helped mobilize 12,000 people in the northeastern city of Dalian to successfully demand the relocation of a petrochemical factory.

"Real name registration is sadly predictable, but very hard to implement, or if implemented is futile anyway as users will just shift to other platforms," said Duncan Clark, managing director of BDA China Ltd., a Beijing research firm.