China extended its internet crackdown today, announcing that it had closed more than 90 websites as part of its campaign to eradicate vulgar and pornographic ­material. But observers fear that the move signals the government's determination to control the net amid a darkening economic outlook and a string of politically sensitive anniversaries.

The authorities are thought to be particularly nervous following the spread of Charter 08, a document calling for political reform including multi-party elections and freedom of expression .

A prominent Chinese blog regarded as a haven for liberal thought and one of the liveliest sites for discussion was also banned last week. Bullog was closed on the grounds that it contained too much "harmful" comment on current affairs. Its founder, Luo Yonghao, said at the weekend that he would reopen it overseas if the authorities did not relent. It was briefly banned in 2007.

The anti-porn drive began with the naming of 33 websites – including Google, MSN China and Baidu, China's biggest search engine – for "vulgar" content or failing to eradicate links to pornographic sites. Although such censorship initiatives are relatively common, it is the first time that officials have targeted such major companies.

Some have read that as a shot across their bows, intended to encourage them to take more care in censoring content in general over the year ahead. This year sees the 20th anniversary of the military crackdown on the Tiananmen Square protests, the 50th anniversary of the failed uprising in Tibet which led to the Dalai Lama fleeing to India, and the 60th anniversary of the founding of the People's Republic.

Late last week the authorities shut 41 sites and today Xinhua said another 50 had been banned – although none of the big names have been closed.

"While the publicly stated purpose of cracking down in the past week has been porn and internet smut, we have also seen the shutdown of Bullog and a number of websites," said Rebecca MacKinnon, assistant professor at the University of Hong Kong and an expert on China and the internet.

"From talking to people who work in web companies here it's pretty clear they feel under increased pressure to control political content as much as smut … I'm being told that all of those companies are beefing up their staff who are employed to police content and the software and other mechanisms to flag content which gets them in trouble."

Xiao Qiang, director of the China Internet Project at the University of California at Berkeley, suggested Bullog's closure reflected the Chinese government's deep concerns about the growing influence of the internet.

"The sheer number of bloggers and the sheer number who are willing to express themselves politically are growing dramatically," he added.

"The language is changing from implicit to more and more explicit, communities are swarming and their opinions and influence are getting stronger – even compared with six months ago."

While Bullog was regarded as pushing limits in online discussion, some believe Charter 08 – which was signed by some of the site's main contributors – may have precipitated a ban.

The Human Rights Defenders network has said that more than 100 of the 300 original signatories had been questioned, detained or harassed by police. The authorities appear to be trying to establish the main movers behind the document.

Despite the clampdown by the authorities, and the vigilance of censors in deleting references to the document, activists say that more than 7,000 people have now signed it following its circulation by email and other means. Several of the original signatories have also defended their decision publicly, insisting that the document is lawful. But there is no question that Beijing is unnerved by the unusual coalition of intellectuals and others.

Nicholas Bequelin of Human Rights Watch added: "There are regular [censorship] campaigns but I do expect the authorities to be particularly vigilant this year about anything published or circulated on the net. They had a warning signal with Charter 08 and I think we are going to see a year of tightening [control] of publication and expression."

http://www.guardian.co.uk/media/2009/jan/13/china-internet-censorship