China has ordered internet microblogs to require users to register with their real names, tightening rules in a bid to control its fast growing social networks.
An announcement posted online said all microblog companies registered in Beijing had to enforce real name registration within three months.
The rules, jointly issued by the government, police and internet management office, apparently apply to all 250 million users of the hugely popular Twitter-like service Weibo.com, regardless of location, because its operator, Chinese web portal Sina Corp, has its headquarters in Beijing.
Sina rival Tencent Holdings is based in the southern city of Shenzhen.
It was not 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.
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 rumours. But the government is also clearly worried about the use of Weibo and other sites to mobilise potentially destabilising 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 mobilise 12,000 people in the north-eastern city of Dalian to successfully demand the relocation of a petrochemical factory and served as an outlet for public anger after a crash on the showcase high-speed rail system in which at least 40 people died.
They also have given a national platform to a handful of independent candidates who have run this year for local councils.
China blocked Twitter and Facebook after they were instrumental in anti-government protests in Iran two years ago, and instead encouraged homegrown alternatives in the apparent belief that domestic companies would be more responsive to government demands.