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.
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.
“Today’s anniversary is one of those ‘red line’ topics that are always subject to a high degree of scrutiny,” Duncan Clark of BDA China told the BBC.
“Typical search results for Chinese search engines of Tiananmen Square return bland descriptions of the square, photos of tourists or the main landmarks, and so on.
“And some are tweeting that the characters for ‘today’ are today banned.”
China’s main microblogging platform, Sina Weibo, has deactivated the candle emoticon, commonly adopted on the web to mourn deaths.
After users responded by trying to replace the banned candle emoticon with the Olympic flame symbol, the website deactivated it too.
When trying to search for the unrest, users have been coming across a message explaining that search results could not be displayed “due to relevant laws, regulations and policies”.
Throughout the years, the government’s methods have been very effective in making people avoid any discussions of the crackdown, added Mr Clark.
“For most Chinese the words ‘Tiananmen Square’ don’t bring to mind the same images and associations as in the West, it’s more like Trafalgar Square to Brits.
