Separately, the Tor Project, a nonprofit group of anticensorship activists, freely offers software that can be used to send messages secretly or to reach blocked Web sites. Its software, first developed at the United States Naval Research Laboratories, is now used by more than 300,000 people globally, from the police to criminals, as well as diplomats and spies.

Political scientists at the University of Toronto have built yet another system, called Psiphon, that allows anyone to evade national Internet firewalls using only a Web browser. Sensing a business opportunity, they have created a company to profit by making it possible for media companies to deliver digital content to Web users behind national firewalls.

The danger in this quiet electronic war is driven home by a stark warning on the group’s Web site: “Bypassing censorship may violate law. Serious thought should be given to the risks involved and potential consequences.”

In this cat-and-mouse game, the cat is fighting back. The Chinese system, which opponents call the Great Firewall of China, is built in part with Western technologies. A study published in February by Rebecca MacKinnon, who teaches journalism at the University of Hong Kong, determined that much blog censorship is performed not by the government but by private Internet service providers, including companies like Yahoo China, Microsoft and MySpace. One-third to more than half of all postings made to three Chinese Internet service providers were not published or were censored, she reported.

When the Falun Gong tried to support its service with advertising several years ago, American companies backed out under pressure from the Chinese government, members said.

In addition, the Chinese government now employs more than 40,000 people as censors at dozens of regional centers, and hundreds of thousands of students are paid to flood the Internet with government messages and crowd out dissenters.

This is not to say that China blocks access to most Internet sites; most of the material on the global Internet is available to Chinese without censorship. The government’s censors mostly censor groups deemed to be state enemies, like the Falun Gong, making it harder for them to reach potential members.