Internet sites containing the words "democracy", "Tibet" and "Taiwan" are among those most frequently blocked by the Chinese government, a study of Chinese net access has revealed.
Researchers at Harvard Law School's Berkman Centre for Internet and Society said that other blocked sites included those on health, education, news, entertainment, religion and pornography.
Ben Edelman, a Berkman researcher, and Jonathan Zittrain, the centre's co-director, found more than 19,000 of the more than 204,000 websites they checked were inaccessible at least some of the time.
Amnesty International last month called for the Chinese government to release 33 people being held for using the internet to circulate or download information.
A report from human rights group estimated that filtering software prevents Chinese internet users accessing as many as 500,000 sites, including many foreign newspapers.
The research also found that the top 10 Google search results using the key words "Tibet", "Taiwan China" and "equality" were all blocked, as were eight of the top 10 search results using "democracy China" and "dissident China". Chinese officials often block an entire website, even if only parts of it contain sensitive information, Mr Edelman said.
It blocks several sites for leading US universities, including Columbia University and the Massachusetts Institute of Technology. Mr Edelman noted that the blocked university sites host campus pro-democracy groups. MIT also hosts scrambling software that makes email unreadable to censors.
Since the research was published, Berkman's site has also become inaccessible in China.
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