Reporters Sans Frontiers and The New York-based Committee to Protect Journalists have also condemned a 'voluntary pledge' made by 130 of China's net-based companies to observe the ban, including Yahoo!
In an open letter to Yahoo! Chairman Terry Semel, Reporters Without Borders said: "Yahoo! has always defended its image as an 'historic' operator in the development of the web; a pioneer fiercely protective of its independence and that of the web sites hosted on its portals - including controversial web sites.
"Reporters Without Borders has, in the not so distant past, defended [Yahoo] - when the controversy over the auction of Nazi memorabilia on one of your sites was in full swing in France. Our organisation did that in the name of the total freedom from which the internet should benefit.
"Reporters Without Borders also fights for the right of Chinese Internet users and operators to enjoy that same freedom. And it has to be recognised that this is not the situation today... (we) ask you to seriously consider revoking this decision and withdrawing your signature, and urge you to reject this practice of self-censorship of the content of your web site."
Content such as pornography, discussion of Tibet and democracy will become illegal under the new legislation.
China has also intensified a crackdown on internet cafes, after 25 people were killed in a fire at an unlicensed establishment in Beijing last month.
The new content legislation follows an announcement from hacker organisations that they plan to help anonymous web surfers by offering free software in countries with tough censorship laws.
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