ABC's Emily MacDonald reports from Beijing:

The Chinese government is not shy about censoring the internet. As tension rose over Tibet before the 2008 Olympics, sites like Youtube, Facebook, Twitter and even Wikipedia found themselves blocked. After the Olympic frenzy, the government seemed to relax and many sites become accessible again. Youtube and most major blog sites, however, were re-blocked this spring when a video of a Buddhist monk being beaten surfaced. Now, in light of the riots in Urumqi, Xinjiang, which the media is calling the next Tiananmen, or the Tibet of 2009, the government is refocusing on internet censorship. The Latest casualty? Facebook. When you try to access Facebook in China, an error message appears saying “Network Timeout.” This means that Chinese residents are without Youtube, Twitter, and other sites where content is predominantly user generated. Interestingly, the Chinese equivalent of many of these sites (i.e., Youku, the Chinese version of Youtube or Xiaonei, the version of Facebook) remain available. One possible explanation for the choice to block major foreign sites and not their domestic equivalents is that the Chinese government might be more concerned about international criticism than internal tension.

Even sites that have not been blocked have been closely monitored. Any content made by either government supporters or Uighur sympathizers that is deemed controversial is immediately taken down by officials. Additionally when you plug in words like Uighur or Xingjiang into popular search engines like Google, sometimes the search engine will abruptly stop working, just to resume about 30 seconds later.

The government has become increasingly aware of methods to override their firewall. Sites that offer downloadable programs such as Freegate and Ultrasurf have been blocked. Other proxy servers are working to stay ahead of government efforts to shut them down, but many have been unsuccessful. In Urumqi, communications security is tight. The internet has been cut, except for a press room which has been provided by the Chinese government. Fifty computers are going to be used to support all of the journalists covering the riots.