4 June 2012 Last updated at 11:12 By Katia Moskvitch Technology reporter, BBC News

Chinese authorities have blocked internet access to search terms related to the 23rd anniversary of the 1989 crackdown against protesters at Beijing's Tiananmen Square.

Terms such as "six four", "23", "candle" and "never forget", typed in Chinese search engines, do not return any information about the event.

Discussions of the unrest of 4 June 1989 remain taboo in the country.

But some users managed to upload a few pictures on to Twitter-like Sina Weibo.

In 1989, troops shot dead hundreds of pro-democracy protesters gathered in central Beijing.

The demonstrations have never been publicly marked in China, and the government has never said how many were killed.

But human rights groups' estimates range from several hundred to several thousand killed.

For most Chinese the words 'Tiananmen square' don't bring to mind the same images and associations as in the West

Analysts say that censoring any online talk related to the event is especially important for Beijing this year, as the government gets ready for a leadership handover.

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China bans Tiananmen web searches