In the last week and a half, China has begun using more sophisticated, expensive technologies in an effort to keep its growing number of Internet users from viewing undesirable content on the Web.
Starting about Sept. 1, users of the U.S.-based search engine Google have found themselves rerouted to a half dozen Chinese-run search engines that are less effective. Google has a feature for finding Chinese-language material online.
A few days ago, users of another American search engine, Altavista.com, also found they were being rerouted to the same little-known, heavily censored Chinese sites.
This week, users have begun complaining of an increase in selective blocking - being able to visit Web sites but not being able to see specific articles or other content of a politically sensitive nature.
A common complaint has been that users can access a foreign media site but find only a blank screen when they try to call up an article on, for example, Chinese President Jiang Zemin.
In its usual secretive way, Beijing made no announcement of the new measures and refused to confirm their existence.
But analysts say the measures represent a technological leap from the earlier "Great Fire Wall" of China, which had clumsily tried to block entire Web sites deemed politically dangerous or pornographic.
"Blocking has been escalated in the last week or so. It's a new high-water mark in Internet controls," said Duncan Clark, managing director of BDA China Ltd., a Beijing-based Internet consulting firm.
Clark and others said the new blocking seems to be related to a Communist Party Congress scheduled for November - a time when restrictions on speech are often tightened.
This congress is especially sensitive because Jiang is expected to give up his post as secretary-general, beginning a process of handing over power to younger leaders.
Ben Edelman, a Harvard University researcher who has been documenting Chinese online censorship, said China's recent filtering modifications "show in new clarity their dedication to restricting access to content they deem undesirable, inappropriate or simply illegal."
Analysts say they're more interested in whether the controls will be eased after the congress.
Pressure to do so will be intense because of the economic costs, analysts said. Installing and upgrading new censorship software is expensive, and the restrictions lead to less comprehensive searches and bog down all Internet use.
On the other hand, authorities may be reluctant to give up their new powers. Chinese Internet companies may also want the restrictions kept in place to block foreign competitors.
"It'll be an early test of the tenor of the new administration" that replaces Jiang, Clark said.
The decision highlights a contradiction at the core of Beijing's Internet policy: it encourages commercial and educational use by China's 30 million-plus users while restricting it as a forum for political discussion.
Many Chinese users are already complaining that the Internet's business utility is being damaged.
"Without the English search engines, users in China are at a dead end," said Ben, a 36-year-old employee at a foreign company in Shanghai who uses the Internet for work. He asked that his family name not be used for fear of official retaliation.
"Chinese engines don't provide sufficient information on Chinese-language sites, let alone English sites," he said.
An operator at the customer service center of Shanghai Online, the largest service provider in China's largest city, said the company has been deluged with complaints.
"Users are quite angry. They rely on foreign search engines as a work tool," said the operator, who spoke on condition of anonymity. "We can only tell customers that the Web sites were shut down by the government and that we can do nothing about it."
A marketing executive at Shanghai Online, who also asked not to be named, said use of its Chinese-language search engine has surged this month.
The government's early blocking of entire sites was technologically fairly primitive and involved placing filters at what were then a handful of international gateways - where China's piece of the Internet merged with the broader global supernetwork.
But gateways are proliferating, and the site-specific blocks are too easily sidestepped by proxy servers, computers abroad that help mask a user's true destination.
The new, selective filtering technologies make it much more difficult to access forbidden information, Clark and others said. Yet they are technologically more difficult to administer - and more costly. Data requests are free to leave China, but many incoming Web pages are blocked based on certain keywords.
"The Chinese have been working on these new technologies for years," Clark said.