Chinese internet users have called for a one-day boycott of cyberspace in protest at a government plan to fit computers with a filter to censor sensitive information.

In a posting on Twitter today, the artist Ai Weiwei —an adviser on the design of the Bird’s Nest stadium built for the Beijing Olympics — appealed for a boycott on July . He wrote: "Stop any online activities, including working, reading, chatting, blogging, gaming and mailing. Don't explain your behaviour."

He hoped that the date, the anniversary of the founding of the Chinese Communist Party, could become a permanent memorial for lack of freedom on the internet in China.

From July 1, all computers produced in China for domestic sale must come with the Green Dam software, that the Government says is designed to block access to pornographic sites and those deemed to be “unhealthy” for young people. The new policy was unveiled 20 days before the deadline, to the consternation of computer companies and outrage among the majority of Chinese who see it an infringement of their privacy and freedom of choice.

Many fear that the software is the start a crackdown on groups and websites that the Government fears or disapproves of.

The US Embassy said that it viewed with concern any attempt to restrict the free flow of information, and was worried about the software's potential impact on trade and the serious technical issues raised by the use of Green Dam.

Mr Ai told The Times: “I hope to awaken national consciousness and action. The constitution regulates that we can strike and so on but in fact we can boycott nothing. We can only boycott our own happiness. I just want to show my view and do the little that I can do.”

The nationwide anger at the latest policy to strengthen the blocks know as the Great Firewall of China has already forced the Government to back down from its initial order that every new computer come with the filter ready installed.

The public has complained that the Government awarded the 41.7million yuan (£4 million) contract to Jinhui Computer System Engineering without any transparent bidding process, instantly making the company the most profitable software manufacturer in the country.

Charges that the software can be exploited easily by hackers have brought a pledge from Jinhui to iron out flaws, but that has failed to mollify many, including parents, who say that the blocks are ineffective — for example, images of the cartoon fat cat Garfield fall foul of the software while pages of graphic sexual pictures can still be accessed.

Reflecting the level of anger, the editor of the influential Caijing business magazine today published a bold commentary attacking Green Dam as lacking validity and moral authority.

Hu SHuli wrote: "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. 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."

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

To add to the controversy, an American company that makes Cybersitter, which allows parents to block pornographic and violent content, has said that the Green Dam programme has been pirated. Solid Oak Software of Santa Barbara, California, has issued “cease and desist” letters to the computer manufacturers Hewlett-Packard and Dell.

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