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.
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.
"This speaks to the efficacy of government controls - many born in that year or after have never heard of what happened, even well-educated university graduates."
The US government has urged China to free all those still in prison after the crackdown.
The US State Department message also called on China to "provide a full public accounting of those killed, detained or missing".
Besides the 1989 unrest at Tiananmen Square, the Chinese authorities censor search terms on the internet that relate to the independence movement in Taiwan, or sensitive postings relating to Tibet, Xinjiang or Communist Party rule.
Although Google search is not banned in China, people using it are routed to the engine's servers based in Hong Kong.
Google in Hong Kong has recently added a new feature indicating to users as they type, in real time, which words are "sensitive".