China has always blocked IP (Internet Protocol) addresses it believes are used by Freegate, which routes users' communication through foreign IP addresses to grant access to Web sites blocked in China. But this month it became more aggressive and began blocking a wider range of IP addresses, risking taking down unrelated targets in order to hit more Freegate users, Xia said. The moves have left most users unable to use the program, prompting Xia's company to ready an updated version of Freegate that will be available in a few days.

China to stifle Freegate ahead of another sensitive date this year: the 20th anniversary of its bloody crackdown on student democracy protests in Beijing's Tiananmen Square in June 1989.

Measures China uses to limit access to certain Web sites include altering entries in the DNS (domain name system), which translates URLs like www.google.com into the numeric IP addresses used to relay information online, and resetting a computer's connection when it tries to visit a banned site. The country's police force also patrols the Internet for sensitive or pornographic content.