The Iranian government, more than almost any other, censors what citizens can read online, using elaborate technology to block millions of Web sites offering news, commentary, videos, music and, until recently, Facebook and YouTube. Search for “women” in Persian and you’re told, “Dear Subscriber, access to this site is not possible.”

Last July, on popular sites that offer free downloads of various software, an escape hatch appeared. The computer program allowed Iranian Internet users to evade government censorship.

College students discovered the key first, then spread it through e-mail messages and file-sharing. By late autumn more than 400,000 Iranians were surfing the uncensored Web.

The software was created not by Iranians, but by Chinese computer experts volunteering for the Falun Gong, a spiritual movement that has beem suppressed by the Chinese government since 1999. They maintain a series of computers in data centers around the world to route Web users’ requests around censors’ firewalls.

The Internet is no longer just an essential channel for commerce, entertainment and information. It has also become a stage for state control — and rebellion against it. Computers are becoming more crucial in global conflicts, not only in spying and military action, but also in determining what information reaches people around the globe.

More than 20 countries now use increasingly sophisticated blocking and filtering systems for Internet content, according to Reporters Without Borders, a Paris-based group that encourages freedom of the press.

Although the most aggressive filtering systems have been erected by authoritarian governments like those in Iran, China, Pakistan, Saudi Arabia and Syria, some Western democracies are also beginning to filter some content, including child pornography and other sexually oriented material.

In response, a disparate alliance of political and religious activists, civil libertarians, Internet entrepreneurs, diplomats and even military officers and intelligence agents are now challenging growing Internet censorship.

The creators of the software seized upon by Iranians are members of the Global Internet Freedom Consortium, based largely in the United States and closely affiliated with Falun Gong. The consortium is one of many small groups developing systems to make it possible for anyone to reach the open Internet. It is the modern equivalent of efforts by organizations like the Voice of America to reach the citizens of closed countries.

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.

Blocking such groups has become more insidious as Internet filtering technology has grown more sophisticated. As with George Orwell’s “Newspeak,” the language in “1984” that got smaller each year, governments can block particular words or phrases without users realizing their Internet searches are being censored.

Those who back the ragtag opponents of censorship criticize the government-run systems as the digital equivalent of the Berlin Wall.

They also see the anticensorship efforts as a powerful political lever. “What is our leverage toward a country like Iran? Very little,” said Michael Horowitz, a fellow at the Hudson Institute who advises the Global Internet Freedom Consortium. “Suppose we have the capacity to make it possible for the president of the United States at will to communicate with hundreds of thousands of Iranians at no risk or limited risk? It just changes the world.”

The United States government and the Voice of America have financed some circumvention technology efforts. But until now the Falun Gong has devoted the most resources, experts said, erecting a system that allows the largest number of Internet users open, uncensored access.

Each week, Chinese Internet users receive 10 million e-mail messages and 70 million instant messages from the consortium. But unlike spam that takes you to Nigerian banking scams or offers deals on drugs like Viagra, these messages offer software to bypass the elaborate government system that blocks access to the Web sites of opposition groups like the Falun Gong.

Shiyu Zhou, a computer scientist, is a founder of the Falun Gong’s consortium. His cyber-war with China began in Tiananmen Square in 1989. A college student and the son of a former general in the intelligence section of the People’s Liberation Army, he said he first understood the power of government-controlled media when overnight the nation’s student protesters were transformed from heroes to killers.

“I was so disappointed,” he said. “People believed the government, they didn’t believe us.”

He decided to leave China and study computer science in graduate school in the United States. In the late 1990s he turned to the study of Falun Gong and then joined with a small group of technically sophisticated members of the spiritual group intent on transmitting millions of e-mail messages to Chinese.

Both he and Peter Yuan Li, another early consortium volunteer, had attended Tsinghua University — China’s Massachusetts Institute of Technology. Mr. Li, the son of farmers, also came to the United States to study computer science, then joined Bell Laboratories before becoming a full-time volunteer.

The risks of building circumvention tools became clear in April 2006 when, Mr. Li later told law enforcement officials, four men invaded his home in suburban Atlanta, covered his head, beat him, searched his files and stole two laptop computers. The F.B.I. has made no arrests in the case and declined to comment. But Mr. Li thinks China sent the invaders.

Early on, the group of dissidents here had some financial backing from the International Broadcasting Bureau of the Voice of America for sending e-mail messages, but the group insists that most of its effort has been based on volunteer labor and contributions.

The consortium’s circumvention system works this way: Government censorship systems like the Great Firewall can block access to certain Internet Protocol addresses. The equivalent of phone numbers, these addresses are quartets of numbers like 209.85.171.100 that identify a Web site, in this case, google.com. By clicking on a link provided in the consortium’s e-mail message, someone in China or Iran trying to reach a forbidden Web site can download software that connects to a computer abroad that then redirects the request to the site’s forbidden address.

The technique works like a basketball bank shot — with the remote computer as the backboard and the desired Web site as the basket. But government systems hunt for and then shut off such alternative routes using a variety of increasingly sophisticated techniques. So the software keeps changing the Internet address of the remote computer — more than once a second. By the time the censors identify an address, the system has already changed it.

China acknowledges that it monitors content on the Internet, but claims to have an agenda much like that of any other country: policing for harmful material, pornography, treasonous propaganda, criminal activity, fraud. The government says Falun Gong is a dangerous cult that has ruined the lives of thousands of people.

Hoping to step up its circumvention efforts, the Falun Gong last year organized extensive lobbying in Congress, which approved $15 million for circumvention services.

But the money was awarded not to the Falun Gong consortium but to Internews, an international organization that supports local media groups.

This year, a broader coalition is organizing to push for more Congressional financing of anti-filtering efforts. Negotiations are under way to bring together dissidents of Vietnam, Iran, the Uighur minority of China, Tibet, Myanmar, Cuba, Cambodia, Laos, as well as the Falun Gong, to lobby Congress for the financing.

Mr. Horowitz argues that $25 million could expand peak usage to as many as 45 million daily Internet users, allowing the systems to reach as many as 10 percent of the Web users in both China and Iran.

Mr. Zhou says his group’s financing is money well spent. “The entire battle over the Internet has boiled down to a battle over resources,” he said. “For every dollar we spend, China has to spend a hundred, maybe hundreds of dollars.”

As for the Falun Gong software, it proved a little too popular among Iranians. By the end of last year the consortium’s computers were overwhelmed. On Jan. 1, the consortium had to do some blocking of its own: It shut down the service for all countries except China.