This is not the first time that foreign companies have been enlisted in government efforts to police the Internet. Google already removes politically forbidden results yielded by its popular search engine, Microsoft allows censors to block content on its blog service, and Yahoo was widely criticized for turning over information that was used to jail a journalist.

“I would advise dissidents to buy computers before July 1,” said Clothilde Le Coz, the head of the Internet freedom desk of Reporters Without Borders.

More than 40 million personal computers were sold last year in China, one of the fastest growing markets. Despite the slowing economy, industry analysts expect that figure to rise by 3 percent this year.

A group of industry representatives met with American officials Monday to express their displeasure with the new rules, said Susan N. Stevenson, a spokeswoman for the United States Embassy in Beijing. “We view any attempt to restrict the free flow of information with great concern,” she said.

Zhang Chenming, general manager of Jinhui Computer System Engineering, a company that helped create Green Dam, said worries that the software could be used to censor a broad range of content or monitor Internet use were overblown. He insisted that the software, which neutralizes programs designed to override China’s so-called Great Firewall, could simply be deleted or temporarily turned off by the user. “A parent can still use this computer to go to porn,” he said.

Although the directive is somewhat imprecise and suggests that manufacturers can provide the software as a compact disc, it also says that it must be installed on computer hard drives as a backup file. The five-point circular uses the word “preinstall” repeatedly and the first clause unequivocally states: “Imported computers shall preinstall the latest available version of the ‘Green Dam’ software before they are sold in China.”

Manufacturers complain that they have had been given little guidance by Chinese authorities. “The wording may be intentionally vague, but the message is clear: we have no choice in the matter,” said one computer executive who spoke on condition of anonymity because some companies are hoping they can persuade the government to ease the requirements.