China is experimenting with more subtle methods to censor Internet search results ahead of the 24th anniversary of the Tiananmen Square crackdown, according to a group that monitors blocked websites in the country.

In the past, a search for keywords in China related to the events of June 4, 1989, came up with an explicit message saying: "According to relevant laws, regulations and policies, search results for (the blocked keyword) can not be displayed."

But GreatFire.org said in the lead up to the anniversary certain searches, such as "June 4 incident", had been intermittently returning a series of "carefully selected results", though it was impossible to click through to the actual webpages.

The organisation said this was an example of "censorship at its worst", with users duped into believing the keyword they were searching for was not a sensitive topic.

Troops killed hundreds of protesters during the pro-democracy protests in Beijing, but GreatFire.org said searches for "Tiananmen incident" returned links to an unrelated happening in the square from 1976.

It said the changes were not applied consistently, concluding that the authorities were conducting tests to improve their control systems.

The Internet in China is purged of politically sensitive websites and Beijing closely monitors the hundreds of millions of web users to prevent organised dissent. Twitter, YouTube and Facebook are banned.

The system of online censorship is dubbed the "Great Firewall", a term combining the words "Great Wall" and computer "firewall".