BEIJING - Ahead of this week's 20th anniversary of the bloody crackdown on Tiananmen Square, Chinese authorities have rounded up dissidents and shipped them out of town. Now, they've even shut down Twitter.

Along with their usual methods of muzzling dissent, the authorities extended their efforts yesterday to silence social-networking sites that might foster discussion of any commemoration of the events of June 3-4, 1989.

The action is a new sign of the government's concern of the potential of such technology in a society where information is tightly controlled.

"There has been a really intensified clampdown on quasi-public discussion of awareness of this event," said Xiao Qiang, adjunct professor of the Graduate School of Journalism at the University of California-Berkeley, and director of the Berkeley China Internet Project.

China has the world's largest online population, and Internet communities have proven increasingly influential in spreading word of events.

People are going outside the normal, controlled channels to set up communities online, spreading information about campus unrest and other activities that the government considers to be potentially subversive.

Government Internet monitors have shut down message boards on more than 6,000 Web sites affiliated with colleges and universities, according to the Hong Kong-based Information Center for Human Rights and Democracy.

Numerous blogs maintained by edgy government critics have been blocked, and both Twitter and the photo-sharing site Flickr could not be accessed yesterday within China.