By BRAD STONE
and DAVID BARBOZA

The Great Firewall of China is hardly impregnable.

Just as Mongol invaders could not be stopped by the Great Wall, Chinese citizens have found ways to circumvent the sophisticated Internet censorship systems designed to restrict them.

They are using a variety of tools to evade government filters and to reach the wide-open Web that the Chinese government deems dangerous - sites like YouTube, Facebook and, if Google makes good on its threat to withdraw from China, Google.cn.

It’s difficult to say precisely how many people in China engage in acts of digital disobedience. But college students in China and activists around the world say the number has been growing ever since the government stepped up efforts to “cleanse” the Web during the Beijing Olympics and the Communist regime’s 60th anniversary last year.

As part of that purge, the Chinese government shut down access to pornography sites, blogs, online video sites, Facebook, Twitter and more.

While only a small percentage of Chinese use these tools to sidestep government filters, the ease with which they can do it illustrates the difficulty any government faces in enforcing the type of strict censorship that was possible only a few years ago.

Jason Ng, a Chinese engineering school graduate who will say only that he works in the media business, wakes every day at 8:30 a.m., and then begins his virtual travels through an open, global network by fanqiang, or “scaling the wall.” He connects to an overseas computer with a link, called a proxy server, that he set up himself. It costs 15 renminbi, or around $2, a month to share with about two dozen other friends.

Mr. Ng then works on his blog and checks the news on Google Reader and Twitter to “officially start my day of information.” Chinese citizens engaged in such practices say the government rarely cracks down on them individually, preferring instead to go after prominent dissidents who publish information about forbidden topics online.

As a result, college students, human rights activists, bloggers, journalists and even multinational corporations in China are rushing to use tools that go over or around barriers set up by Chinese regulators, in part because they feel it is the only way to participate in a global online community.

Isaac Mao, a well-known blogger and activist in China, says the number of people seeking access to blocked sites has grown as popular Web sites have been shut down by Beijing.

These digital dissidents have begun to organize small conferences and networks to share information and tricks about how to get access to banned material. “People start to hold a grudge against the government for depriving them of access to the Web sites they regularly visit,” Mr. Mao says.

But as the government has expanded its control over Internet, it has also intensified efforts to close some of the channels being used to evade the online blockade.

AnchorFree, a start-up based in Sunnyvale, California, has built a profitable business by providing free, advertising-supported software called Hotspot Shield that tunnels about 7.5 million people around the world into the Internet by encrypting Internet users’ data and cloaking their identities.

But last summer, the Chinese government blocked AnchorFree’s Web site so that Chinese citizens could no longer download the software. Almost immediately, its users began e-mailing their own copies of the program to friends and posting links to other sites that hosted it. The program’s use in China has doubled since then, said David Gorodyansky, AnchorFree’s founder.

Despite these bureaucratic battles, people on the side of greater Internet freedoms in the continuing fight against Big Brother say the battlefield is inherently tilted in their favor.

“The architecture of the Internet makes our work easier,” said Bill Xia, a programmer based in North Carolina whose software tools, including DynaWeb and FreeGate, are used by hundreds of thousands of people in China every day to access forbidden sites.

“The starting point of the Internet is open networks. Everybody can publish and receive data, and unless they want to shut down the whole Internet, we have the advantage.”

Some Chinese use outside Web services to thwart government censorship. A cafe in Shanghai. / PHILIPPE LOPEZ/AGENCE FRANCE-PRESSE— GETTY IMAGES