HONG KONG — If an Internet user in China searches for the word "persecution," he or she is likely to come up with a link to a blank screen that says "page cannot be displayed."

The same is true of searches for "Tibetan independence," "democracy movements" or stranger sounding terms such as "oriental red space time" — code for an anti-censorship video made secretly by reporters at China's state TV station.

It's a reflection of the stifling, bizarre and sometimes dangerous world of Internet censorship in China. The communist government in Beijing is intensifying its efforts to control what its citizens can read and discuss online as political tensions rise ahead of this summer's Olympic Games.

Fighting the censors every step of the way is an army of self-described "hacktivists" such as Bill Xia, a Chinese-born software engineer who lives in North Carolina. Xia and others are engaged in a kind of technological arms race, inventing software and using other tactics to allow ordinary Chinese to beat the "Great Firewall of China" and access information on sensitive subjects such as Chinese human rights and Tibet, the province where pro-independence sentiment has boiled over in recent months.

Invoking the hit science-fiction movie The Matrix, Xia has compared what he does to giving Chinese Web surfers a "red pill" that lets them see reality for the first time. He spends long nights struggling to outfox an opponent — the Chinese government — that is arguably the world's best at controlling what its people see.

"They are very smart," Xia says. "We have to move very quickly."

To Americans and other Westerners, it might seem odd that Internet censorship is still possible at a time when YouTube, satellite TV and online chat rooms produce an overwhelming flow of real-time news and data. Yet authoritarian regimes from Cuba to Saudi Arabia to Pakistan rely on a mix of sophisticated technology and old-fashioned intimidation to ensure that dissent can be repressed, even in the Information Age.

No one does it quite like China, which has proved that old-school communist apparatchiks could tame something as wild as the Web. China has the world's "most sophisticated" Internet filtering system, according to the OpenNet Initiative, an academic cooperative that tracks censorship issues.

At the heart of China's censorship efforts is a delicate balancing act.

Unlike communist North Korea, which bans online access to its general population, China is encouraging Internet usage as it rushes to construct a modern economy. This year, the number of Internet users in China surpassed the USA for the first time, hitting 233 million by the end of March. However, China's government does not tolerate opposition and is wary of the variety of views and information the Web brings.

Last month's pro-independence riots in Tibet, and the accompanying furor that followed the international relay of the Olympic torch, have led Chinese officials to step up their Web censorship. News articles and video clips concerning Tibet were banned for several days. Xia expects the censorship will tighten further in the coming months because "many human rights organizations will be trying to get their voices heard" during the Olympic Games.

"There will be lots of news out there," says Xia, who admits he had little interest in politics until the Chinese government banned the spiritual group Falun Gong in 1999 and started persecuting its members. Xia is a member of the group.

"Lots of unexpected things are going to happen," he says.

Forbidden words, stories

The most basic tool at the Chinese government's disposal — and, perhaps, the one most easily circumvented by dissidents — is to ban access within China to websites such as Voice of America or to certain stories that contain sensitive words and phrases. For example, several recent USA TODAY stories about Tibet are currently blocked within China.

Other censorship methods are more blunt. This month, Hu Jia, an activist on AIDS and other issues, was sentenced to 3½ years in jail for articles he wrote for Boxun.com, a U.S.-based Chinese-language website that is banned in China. At least 48 cyberdissidents are behind bars in China, according to Reporters Without Borders.

Chinese officials with the Ministry of Information Industry, the State Council Information Office and other agencies declined to comment on why China restricts content on the Internet.

Past explanations by the government focus on the need to prevent "harmful" content such as pornography and terrorism from reaching citizens.

Even those "hacktivists" who live outside the country apparently face risks. Peter Li — a Chinese-born, Princeton-educated computer specialist — says he learned that two years ago when he answered the doorbell at his home in suburban Atlanta.

Three men burst inside, beat him, bound him and gagged him with duct tape, he says. Speaking Korean and Chinese, they ransacked his filing cabinets and hauled off his two computers. They ignored a TV, a camcorder and other valuables.

The FBI and the local Fulton County, Ga., police still have not found the men responsible for the attack. But Li, who like Xia is a practicing member of Falun Gong, says it was an attempt by the Chinese government to shut him up.

"I know it wasn't a simple robbery," he says.

The Chinese government has denied any involvement in the raid on Li's home.

There are a range of other methods China has used to suppress information. Among them:

•Creating bottlenecks. In The Atlantic magazine last month, journalist James Fallows noted that Internet traffic to China is channeled through three computer centers — near Beijing, Shanghai and the southern city of Guangzhou.

In the USA, by contrast, the Internet is designed to avoid traffic jams by allowing information to flow from as many sources as possible. By building in chokepoints, Fallows wrote, "Chinese authorities can easily do something that would be harder in most developed countries: physically monitor all traffic into or out of the country."

•Checking Internet traffic for subversive material. This is done in much the same way police dogs sniff airport luggage for illegal drugs. The Chinese install "packet sniffers" and special routers to inspect data as they cruise past the chokepoints. If the detectors spot a Chinese Internet user trying to visit a suspect website — say, one run by Falun Gong — they can block the connection.

A frustrated user might get a message saying: "Site not found." Similarly, Web users can be stopped from leaving subversive comments in online forums. Sometimes they get notes back warning them to behave or apologizing for technical problems.

•Demanding self-censorship. Chinese authorities hold commercial websites responsible for what appears on them. In Beijing — where Internet controls are strictest — authorities issue orders to website managers through cellphone text messages and demand that they comply within 30 minutes, according to a report last fall by Reporters Without Borders.

When the Internet portal Sina altered the headline of a state media report on the economy, the government accused it of "inciting violence" and excluded it from interviews with important officials for a month. The website NetEase fired two editors after they published a 2006 poll showing that 64% of 10,000 participants would not want to be reborn as Chinese.

•Issuing propaganda. Authorities in the southern boomtown of Shenzhen created two cute cartoon cybercops — the male Jingjing and the female Chacha — that pop up on websites to remind Internet users they're being watched. The Beijing Youth Daily newspaper quoted a security official admitting that the big-eyed cartoon duo were designed "to intimidate."

Chinese officials also order websites to reprint official propaganda such as a report encouraging Internet users to abide by online etiquette.

•Getting outside help. China has policed the Internet with assistance from U.S. firms. Cisco Systems, for instance, supplied the original routers China used to monitor Internet traffic. (Cisco has said it didn't tailor its equipment for the Chinese market.)

Google created a censored search engine for China. Outside China, users who search Google Images for "Tiananmen Square" get pictures from the 1989 pro-democracy protests that ended in a crackdown that left hundreds dead — and included the iconic photograph of a lone man staring down a line of Chinese tanks. Inside China, users get only tourist images of Tiananmen Square and the Forbidden City across the street.

Yahoo turned over e-mail that authorities used to jail a Chinese journalist who leaked information about China's attempts to censor coverage of the anniversary of the Tiananmen crackdown.. (The companies say they had to comply with Chinese law.)

Despite China's strategies, sophisticated Internet users in the country "can pretty much get as much information as (they) want," says Jeremy Goldkorn, the Beijing-based editor of the China media website danwei.org. "But what (the government does) is make it difficult, so the ordinary person is not going to bother."

Censorship loopholes

In 2002, Xia formed a company — Dynamic Internet Technology — to wage cyberwar on the Chinese regime. He created Freegate, a software program that finds holes in the firewall and takes Chinese Internet users to banned websites, undetected.

Xia also sends millions of e-mail messages into China for customers such as Voice of America and the activist group Human Rights in China. The e-mails contain links to forbidden sites at an ever-changing list of temporary Internet addresses, part of an effort to stay a step ahead of Chinese censors.

Traffic on his network of "proxy" websites picked up in February, when heavy snows blocked traffic and shut train service in southern China, Xia says.

The Chinese government was reluctant to admit anything had gone wrong, so frustrated travelers turned to renegade websites to get practical information on weather conditions and rail service.

Even so, Chinese authorities constantly are finding new ways to plug the holes Freegate finds or to otherwise stymie Xia's efforts. He figures has upgraded Freegate 20 times. "We're gradually getting faster and faster" at fixing problems with the software when Chinese users report them, he says.

Chinese Internet users also use decidedly low-tech methods to evade official attempts to censor their e-mail or online commentary.

They will, for instance, try to throw off the cybercops by inserting spaces or punctuation marks between characters — much as spammers in the USA try to beat e-mail filters by offering "Free V i@gra!"

The authorities try to update their list of banned terms — now running into the hundreds — to include those with creative punctuation.

Rebecca MacKinnon, former Beijing bureau chief for CNN, spotted the way some cheeky Chinese Internet users stayed ahead of the censors. Whenever their edgy comments were purged from a website, they'd joke online that they'd "been harmonized" — a sarcastic reference to Chinese President Hu Jintao's calls for a "harmonious society." Soon, the censors caught on and added "harmonized" to the blacklist.

The Chinese term for "harmonized" is he xie— which sounds the same as the Chinese term for "river crabs" but with a slightly different intonation. Now, Chinese online chatter frequently includes references to river crabs — the latest code for censorship, says MacKinnon, who studies the Chinese Internet at the University of Hong Kong.

Xia says he's confident the "hacktivists" can win their cat-and-mouse game with the Chinese authorities. After all, he says, the Chinese zodiac favors rodents in 2008: "It's the Year of the Rat."

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