Dozens of blogs by some of China's most outspoken users have been abruptly shut down while popular Twitter-like services appear to be the newest target in government efforts to control social networking.

More and more Chinese bloggers are using the newer microblogs as their primary publishing tool, using their brief, punchy message format to chat with one another and promote their longer blog posts.

But one of the country's top four microblog sites is now down for maintenance, and the other three show a "beta" tag as if they are in testing, though they have been operating for months. The companies that run the websites aren't saying why.

"I was writing a new post and suddenly my blog couldn't open," lawyer Pu Zhiqiang told The Associated Press.

Legal expert Xu Zhiyong said his blog on the popular Sohu portal was also shut down Wednesday, a day after his Sohu microblog was closed.

Both men are well known for taking on sensitive issues.

Chinese officials fear that public opinion might spiral out of control as social networking — and social unrest — boom among its 420 million internet users. China maintains the world's most extensive internet monitoring and filtering system, and it unplugged Twitter and Facebook last year.

Blogger Yao Yuan listed at least 61 closed Sohu blogs, including his own, on a separate, unblocked blog Thursday. He called the closings mass murder.

Controls increasing

"If internet users don't speak out, all sites will be cracked down on in the future," said Yao, who owns an internet-promotion company in Shanghai. "Ordinary people will forever lose their freedom to speak online, and the government can rest without worrying anymore."

Microblogs can quickly aggregate critical voices, which is why authorities have been increasing controls, said Xiao Qiang, director of the China Internet Project at the University of California-Berkeley.

"However, given the speed and volume of microblogging content produced in Chinese cyberspace, censors are still several steps behind at this stage," he said in an e-mail.

China's government actually embraced microblogs earlier this year, with the Communist Party newspaper, the People's Daily, launching a microblog of its own.

The People's Daily microblog showed no sign Thursday of new restrictions. Meanwhile, Beijing's public security bureau announced it would set up a microblog for the city's police, the state-run Xinhua News Agency reported Wednesday.

But in April, a leading internet regulator called for requirements that people use their real names when going online.

"As long as our country's internet is linked to the global internet, there will be channels and means for all sorts of harmful foreign information to appear on our domestic internet," Wang Chen, director of the State Council Information Office, said in comments released this week by the New York-based group Human Rights in China.

'Weak links"

"Many weak links still exist in our work. These problems have weakened our ability to manage the internet scientifically and effectively."

Privately run microblogs are showing signs of feeling pinched. The Netease.com microblog is down for maintenance, while the Sina, Sohu and Tencent microblogs display a beta tag.

Sina president Chen Tong responded Wednesday night to speculation that the site could be shut down. "Of course not," he said on the site's microblog. "I've said that sentence more than any other one today."

Government officials could not be reached for comment.

Despite Beijing's extensive restrictions, technologically savvy users can still jump China's "Great Firewall" with proxy servers or other alternatives. And they can just keep publishing. Pu, the lawyer, said he has already set up a new Sohu blog — his 13th so far.