BEIJING, CHINA: Chinese Internet users are calling on fellow web surfers to stay offline on July 1, the debut of a controversial software filter that critics say the Chinese government is using to tighten censorship.

New regulations from Beijing mandate "Green Dam," a program sold by Jinhui Computer System Engineering Co., be pre-installed on personal computers manufactured or shipped after July 1. China says the filter is designed to block pornography.

But many web users and activists both inside and outside China fear a campaign against "unhealthy" sites is a pretext for a wider crackdown on groups and websites that the government fears or disapproves of.

The U.S. embassy said it viewed with concern any attempt to restrict the free flow of information, and was worried about both the potential impact on trade of the software and the serious technical issues raised by the use of Green Dam.

Outspoken Beijing artist Ai Weiwei, who helped design the landmark Olympic Bird's Nest stadium but has become an increasingly vocal critic of the government, called on web users to boycott use of the Internet on the day of Green Dam's debut.

In a post on Twitter (www.twitter.com), Ai called for the low key protests to mark a day that is also the anniversary of the founding of China's Communist Party.

"Stop any online activities, including working, reading, chatting, blogging, gaming and mailing," Ai wrote in the Chinese-language post. "Don't explain your behaviour."

Ai told Reuters he hoped the boycott would gather support because it offered an easy way to make a stand in a country where vocal opposition to government policy can be risky.

"It's an online protest without any cost or risk," Ai told Reuters in an email. "It aims to oppose Internet censorship."

"I haven't counted the number of supporters, but there are many of them," he added.

His message has already been picked up and passed on by other Internet users backing the call for a day offline.

"Lacking Morality"

The introduction of Green Dam has raised concerns from industry as well as rights groups, ranging from worries about compatibility, intellectual property rights and support for the software to cyber-security and Internet freedoms.

Initial criticism of the software in Chinese media has been muted in recent days, but the editor of the influential Caijing business magazine on Monday published a commentary slamming Green Dam as lacking validity and moral authority.

"In order to prevent the transmission on the Internet of violence, and of vulgar information that harms young people ... there must be some form of public authority backing up social rights," Hu Shuli wrote.

"But the help should be a kind of service, must not be coercive, and should have the acceptance of society. Otherwise ... it will certainly be thwarted," she said.

One web survey published in the Beijing Times showed most respondents thought the software violated privacy and were not willing to pay for it once a free one-year subscription expired.

The rollout of Green Dam has come on the heels of a wider, long-running campaign to "clean up" the Internet.

The government last week condemned the Chinese-language version of Google and several domestic sites, including high profile Mop and Tencent, for "disseminating pornographic and vulgar information" and asked them to remove some content.

Analysts say the Communist Party is fighting to stifle dissent in a year of sensitive anniversaries, including the 20th anniversary of the June 4 crackdown on pro-democracy protesters.