Social-networking sites Twitter, Flickr go dark in China

— -- Some of the world's most popular networking services have gone dark in China, apparent victims of government censors in the days leading to a notorious anniversary.

Online users in China said Twitter, Yahoo's Flickr photo site, Microsoft's new Bing search engine and Hotmail, and other services were inaccessible on Tuesday.

This week marks the 20th anniversary of the bloody military crackdown on pro-democracy protesters at Tiananmen Square. Social media experts such as Laura Fitton suspect Chinese authorities may be blocking sites to tamp down discussions of the protest.

If so, it would be the first time Twitter has been censored by the Chinese government. The microblogging service had not been subject to restrictions even though it's used by Chinese activists, who often use aliases.

The Chinese government has not issued an explanation.

"We understand the Chinese government is blocking access to Flickr and other international sites," Flickr spokesman Jason Khoury said in a statement.

Added Microsoft spokesman Kevin Kutz: "We are reaching out to the government to understand this decision." Twitter officials said they're looking into the situation.

Chinese authorities have tightened surveillance of dissidents ahead of the anniversary. Now they're extending restrictions to social-networking services in a society where information is tightly controlled. Those sites have been effective in spreading the word on student protests.

Government officials have shut down message boards on more than 6,000 websites affiliated with colleges and universities, says the Information Center for Human Rights and Democracy.

Video-sharing site YouTube has been blocked within China since March, according to Google, which owns the service.

"Authorities make a point of locking down public discussion this time of year — especially tools like Twitter and Flickr that could be used to organize protests," says Danny O'Brien, international outreach coordinator at Electronic Frontier Foundation, a non-profit.