BEIJING -- The illicit e-mail seemed tantalizingly close, glowing quietly in a Hotmail inbox in Beijing. But a click on it produces nothing except an ominous beep and an error message. And then the entire inbox seizes up, freezing helplessly until the computer is rebooted.

The long arm of the Chinese law has just reached inside another computer, wreaking electronic havoc on a hapless user.

The shutdown is triggered when a high-tech "packet sniffer" detects an illicit word in a message. Perhaps it is a word about a banned religious sect or a separatist region, or perhaps a phrase about China's Internet police tactics. There is never any official explanation of the illegal words.

The computer is swiftly shut down, and China's Web surveillance team has scored another victory.

Until recently, Internet boosters were convinced that their technological marvels would unlock the doors to greater freedom and democracy in police states such as China. But in the past few weeks, with a dramatic escalation in blocking and filtering Web sites and e-mail, Chinese authorities are demonstrating that their so-called Great Firewall can thwart almost any unauthorized ambitions among the country's 50 million Internet users. China's controls have proven much more powerful than anyone expected.

First came a crackdown on Internet cafés this summer, after a fatal fire at a Beijing establishment. About 14,000 were closed across the country. Several thousand were not permitted to reopen, and the rest were forced to install devices to block unauthorized sites.

In August came a set of regulations requiring Web-site operators to censor their political content.

Then, early last month, the popular Google and AltaVista search engines were abruptly shut down in China, an unprecedented step. They were eventually permitted to function again, but this time with a key change: Google's cache function, which had allowed Chinese users to see some banned Web pages, was permanently disabled.

Shortly afterward, China introduced sophisticated new software that seemed capable of "sniffing" for illicit keywords and freezing any e-mail or Web sites that contained the words. Until this year, Web-based e-mail sites such as Hotmail and Yahoo appeared to be evading the filtering software. But now the electronic police are prying inside those havens too. Any e-mail or Web site downloaded through Chinese servers is vulnerable to filtering or blocking. And even when a Web site is left accessible, China has now developed technology to block specific pages within the site.

"They are definitely getting slicker about it," said a California consultant on Internet security who asked not to be named. He said the Chinese technology now seems advanced enough to examine every request for a Web page.

Internet-based dissidents have been playing a cat-and-mouse game with the Chinese government for years. More than 30,000 state employees have reportedly been assigned to watch the Internet, blocking sites and monitoring e-mail messages and chat rooms. In the past two years, at least 25 Chinese citizens have been arrested for using the Internet to spread "subversive" messages.

Internet experts in Western countries were convinced that Chinese surfers could bypass the censorship with "proxy servers" that the government couldn't detect. Instead, the authorities have become faster and faster at finding the proxies, using "proxy hunter" technology and other tools. Now the proxies are almost useless.

Paradoxically, the Chinese government has recognized that the Internet is crucial to China's industrial future.

It has spent billions of dollars on the latest information technology, unleashing an army of young engineers to create new products.

Much of this energy, however, is devoted to exploiting these products for political goals: controlling the Internet, blocking sites and launching high-tech hacker attacks on Beijing's enemies, especially the Web sites of the Falun Gong religious sect and the Tibetan pro-independence movement.