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In this photo taken on May 29 a police car is parked as officers monitor visitors at Tiananmen Square in Beijing. ((Andy Wong/Associated Press))

The Chinese government is trying to block access to social media, photo-sharing and many other websites in an apparent effort to control discussion about the Tiananmen Square pro-democracy protests, bloodily suppressed by Chinese soldiers 20 years ago.

The blocks on Tuesday included Twitter, some blogs, Hotmail, Microsoft's new search engine Bing and Flickr, media reports said. YouTube has been blocked since March.

As many as 6,000 message boards at colleges were blocked in late May, the Information Center for Human Rights and Democracy said in a report dated May 26, but covered by Western media Tuesday. It's based in Hong Kong.

China has repeatedly tried to block electronic access to material it deems illegal under its Great Firewall policy. It bans certain news about Tibet, democracy, anti-government or anti-Communist party activities and sexual content.

The Tiananmen demonstration, where students and some politicians sought more democracy, was crushed by troops and tanks in the square in Beijing on June 4, 1989. Many were killed, but the precise toll has never been revealed. The government calls the demonstrations "counterrevolutionary" riots.

If Chinese internet users enter "4 June" in the photo section of Baidu, the country’s most popular search engine, they get a message saying:  "the search does not comply with laws, regulations and policies," Reporters without Borders said in a news release Tuesday.

Twitter messages were encouraging people to wear white, the colour of mourning, until they were suddenly blocked Tuesday, the Globe and Mail reported.

However, some users have found workarounds that enable them to continue using Twitter, email and websites.

For example, writer williamlong complained on twitter about the block after it was launched, the Guardian newspaper said. "We netizens were beaten by a 'combination blow.' So many famous websites are not accessible now," williamlong wrote.

"Despite China’s heavy-handed internet filtration regime, the large quantity of proxy software allows end-users to circumvent the government-imposed firewall," a recent blog posting on the OpenNet Initiative said. The initiative, rooted in Harvard University, Oxford University, Cambridge University and the Citizen Lab at the University of Toronto, monitors political filtering.

With files from The Associated Press