Ahead of the 20th anniversary of the bloody crackdown on Tiananmen Square this week, 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 Tuesday 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 an authoritarian society where information is tightly controlled.