BEIJING—These appear to be dark days for the Internet in China.

Four months into a crusade against Internet pornography, the government is closing thousands of sites—some pornographic, some not—and tightening rules on who can register Web addresses inside China.

Customers surf the Web at an Internet café in Wuhu, central China, in a photo taken in February. A backlash against Beijing's moves to block access to the Internet has spurred attempts by many users to 'scale' the so-called Great Firewall of censorship. Agence France-Presse

Foreign sites such as Facebook, YouTube and Twitter, blocked by censors in the run-up to the 60th anniversary of Communist Party rule on Oct. 1, remain inaccessible to most Chinese users. Several prominent critics of the state who used the Internet to spread their message have been detained or imprisoned.

Yet this list of casualties obscures a larger truth: The censors are losing.

The dozen or so years since the Web came to China have seen repeated rounds of crackdowns and detentions, aided by a steady growth in scope and sophistication of the government's filtering apparatus that critics dub the Great Firewall. Still, the Internet has enabled more Chinese to have more access to information today, and given them greater ability to communicate and express themselves than at any time since the founding of the People's Republic.

The censors "are winning the battles everywhere," says Isaac Mao, a blogging pioneer based in China and Chinese-Internet researcher, "but losing the war."

In 2009, Beijing lost a big battle, too, in the so-called Green Dam episode. It was the most dramatic illustration of the limits of the censors' power. The government's plan to quietly compel all personal-computer makers put Web-filtering software known as Green Dam-Youth Escort into new PCs shipped into China was indefinitely shelved, amid anger from global technology companies and Chinese citizens alike.

The government said the software was meant to block children from accessing pornography, but critics said that it was unreasonable to require a specific program for all PCs, and that the software was filtering a broad range of content, such as social and political commentary, and even health, among others.

What would have been the state's most extensive measure ever to cleanse the Web instead awakened a new segment of society to the constraints imposed on them. The Great Firewall's power used to be in the government's ability to keep its vast Internet control system under the radar of Chinese users, few of whom use the Web mainly for politics.

Now, "fan qiang"—a cyber dissident's phrase meaning to "scale the wall"—has become standard lingo for Chinese Internet users of many persuasions.

This year, the domestic backlash against Green Dam spread through the Internet, as did much lively discussion over matters long off-limits for public debate. It carried word of a young woman prosecuted for the self-defense killing of a local-government official who had tried to rape her.

In another case, it spread awareness that officials blamed the death of a man in police custody on a game of hide-and-seek with other inmates that turned deadly, which in turn led to accusations by Internet users of a cover-up. A relatively small—and growing—group of savvy Internet users have been able to able to access blocked social networking sites such as Twitter to express defiance over Beijing's Web restrictions and to share banned information.

More broadly, the Internet has given citizens a chance to discuss and organize action on sensitive issues.

"The Internet has been very important. You can express yourself; you can distribute information to change other people's views; you can communicate; you can organize," says Wan Yanhai, a prominent Beijing-based AIDS activist, who started his organization with the help of email and the Web. "In the past 10 years, it has affected people's lives so much. It has given people courage to change society."

To say that the censors are losing isn't to say they have lost. If the Communist Party's grip over information is loosening, it is far from clear whether its hold on political power in China is ultimately threatened by the trend.

To the extent authorities allow more freedom to vent on the Internet, they may even help preserve party power by providing a necessary release valve for complaints.

The Communist Party has always been acutely aware of the power of information. From the start of its rule, it barred foreign news sources, and propaganda officials tightly controlled the content of every publication and broadcast in the country. A brief period of liberalization came in the late 1980s, when college students and other members of the elite were allowed greater leeway to gather and discuss ideas. But that freedom was limited by technological and other constraints on the spread of information. The period ended with the government's crackdown on the Tiananmen Square pro-democracy demonstrations in 1989.

Less than a decade later, the Web's advent in China posed a conundrum for the party that has never gone away. Officials recognized the Internet's utility as access to the outside world and a tool for commerce, even if they fretted about its risks. From the start, they shaped plans to control it. In 1996, they said all Internet subscribers had to register with their local police bureau—a mechanism that might have seemed feasible when there were less than one million users, but quickly became untenable as the number grew to the tens, and then hundreds, of millions.

In 2003, China announced a large-scale plan to regulate the Internet called the Golden Shield Project, with the expressed purpose of letting public-security officials do online monitoring.

Today, multiple government agencies oversee a slew of Internet controls that include regulating locally run Web sites and forcing them to filter out illegal content, such as pornography or sensitive political topics. These agencies might ask sites to provide information on users, may block overseas sites with sophisticated keyword-filtering technology, or at times even attempt to sway public opinion by planting comments on various Internet forums.

The government took more drastic measures when ethnic violence erupted in Xinjiang in July, and panic spread in part through rumors dispersed in text messages and in social media that a spate of syringe stabbings were an effort by Uighurs to infect Han Chinese with HIV. Beijing blocked Internet access in the entire province. On Tuesday, officials announced the blocks would be partially lifted, with access restored only to the Web sites of two state-run media agencies.

The vast majority of people in China use the Web for entertainment, not unlike what people elsewhere do: playing games, listening to music, getting celebrity gossip or reading about sports. That trend is encouraged by Beijing's efforts to curtain off certain subjects.

Web users in China who gain too much attention or strike at especially sensitive subjects are sometimes jailed. That's what happened to Zhao Lianhai. After his young son was sickened by tainted milk in 2008, he started a Web site to help other families and share experiences.

From a dimly lighted office in his home, Mr. Zhao compiled information from around the country into a database of children affected by the tainted formula, and published it on his Web site. He kept his instant-messaging program open at all times to keep in touch with dozens of parents to track lasting effects of melamine poisoning, and to remind them to submit medical records for his database.

Running the site and getting past government barriers on the Internet became a full-time activity for Mr. Zhao, who stopped running the advertising company he had before his son got sick. Mr. Zhao learned to outsmart China's censorship system by moving his site to different servers, using special software that circumvents government filters and registering Web domains outside of China.

"I'm not doing anything wrong, and I say that to anyone who tries to stop me," Mr. Zhao said in an interview during 2009.

Mr. Zhao's activities so alarmed officials that they detained him in November, and formally arrested him in December.

But for each critic the authorities stop, more rise. "There are simply too many people," says Xiao Qiang, a scholar who studies the Chinese Internet at the University of California at Berkeley. "They can do that to a very small group … but the approach certainly is not good enough to intimidate all the voices online."

Mr. Xiao points to the example of Liu Xiaobo, detained in December 2008 for his role in creating Charter 08, a sweeping call for political and legal reform in China. Mr. Liu was sentenced on Christmas Day to 11 years in prison for subversion. But since his detention, thousands more Chinese have signed Charter 08 through Internet sites that disseminate the document.

The government is getting better and faster in its effort to control content on the Internet, but it simply can't keep up with the proliferating moves to use the Web in more ways. In the first six months of 2009, an average of 220,995 Chinese a day started using the Internet for the first time, according to official figures. That represents 153 new Internet users a minute.

That the Internet threatens, fundamentally, the party's information monopoly is one of the few facts that China's liberal activists and its government enforcers agree on. In an essay published in December in a government magazine, Minister of Public Security Meng Jianzhu warned that the Internet "has become an important means for anti-China forces to engage in infiltration and sabotage, and to enlarge their power of destruction, which brings new challenges to the public security agencies to maintain national security and social stability." He pointed to the use of the Internet to spread word of unrest before the government has a chance to control it.

For Mr. Xiao at Berkeley, "essentially, the Internet is mainstream media. Whatever happens on the Internet, the whole nation knows, and that also gets on the government's nerves."

From his perch in California, Mr. Xiao and his team spend most of their time scanning the Chinese Web, and documenting numerous cases of dissent and criticism.

Censorship is "more sophisticated, and its capacity is very powerful, but it is full of loopholes," he says. As the government tries to close them, "the main result is to create more resistance and backlash from Chinese Internet users," Mr. Xiao says. "They are creating a whole lot more enemies to the censorship system."

Write to Loretta Chao at loretta.chao@wsj.com and Jason Dean at jason.dean@wsj.com