Besides Tiananmen, this year is the 10th anniversary of China banning Falun Gong, a spiritual movement, and the 60th anniversary of China's founding, which Beijing will mark with a big military parade.

Dissidents, including many under house arrest and constant monitoring, have turned to tools like proxy servers and Skype to communicate with each other and the world outside China. But while Skype encrypts calls and instant messages, the only version available on the Chinese Internet comes from a joint venture between Skype and a Chinese portal. That version uses keyword filtering to block messages with sensitive content, which it then stores along with user data, researchers at the University of Toronto said in last year.

China this year also redoubled its efforts against at least one popular program used to circumvent its Internet filtering. Chinese users of the program, called FreeGate, began reporting problems including slower loading of foreign Web sites early this year, said Bill Xia, president of Dynamic Internet Technology, the developer of the software.

Hundreds of thousands of people use FreeGate each day, including many dissidents, said Xia. The program encrypts users' communication and routes it through IP (Internet Protocol) addresses abroad, granting access to Web sites blocked in China.

Chinese censors have long tried to identify encrypted FreeGate traffic so they can block the foreign IP addresses channeling it, said Xia. Users are given a new IP address when that happens, but this year China's IP blocking became faster and more aggressive, Xia said.