A Chinese newspaper has fired three editors for failing to censor a one-line classified advertisement that paid tribute to the mothers of protesters killed during the 1989 Tiananmen Square massacre.

Breaking one of the state's strongest political taboos, the obscurely worded advertisement was slipped into the Chengdu Evening News on Monday, the 18th anniversary of the bloody crackdown.

Its publication shows how commercialism, ignorance and technology have created chinks in the one-party state's block on information.

The tiny advertisement, on page 14 of the paper, said: "Saluting the strong mothers of victims of 64."

Six-four (June 4) is the most commonly used expression for the crackdown on that day in 1989, when hundreds, possibly thousands, of pro-democracy demonstrators and their supporters were killed by People's Liberation Army tanks and troops.

The government insists the actions were necessary to restore order, but it has blocked all public debate on the issue and continues to hold some of the protesters in prison.

Despite, the ban, several mothers of the victims have defiantly called for an open investigation of the killings.

But some young Chinese have little or no knowledge of the massacre. According to the South China Morning Post, a young female clerk - responsible for vetting ads in the Chengdu Evening News - allowed the tribute to be published because she was unaware of the significance of 64. When she phoned the person who placed the ad to ask, he reportedly told her it was the date of a mining disaster.

The daring message was quickly spotted, scanned and circulated on the internet before the authorities intervened. Copies of the newspaper were withdrawn from kiosks and an investigation team was sent to the paper's headquarters in Sichuan's provincial capital.

According to Reuters, the deputy editor-in-chief, Li Zhaojun was fired along with two other employees. Staff at the newspaper, which is a 32-page tabloid with a circulation of 200,000, refused to comment when they were contacted by the Guardian.

Free speech campaigners say the editors are being punished to deter others from loosening their guard on the communist party's information wall.

"It shows the government will do everything to stop any reference to this issue appearing in the media," said Vincent Brossel of Reporters Without Borders. The censorship is so strong that the young editor had no historical knowledge about June. It shows how the government is trying to remove the memory of the Chinese people."

Despite economic liberalisation and the increasing commercialisation of the media, the state maintains a tight grip on content. The propaganda department sends out weekly lists of stories that are not allowed to be published or broadcast.

Coverage of contentious topics - such as Tiananmen, Taiwan, Tibet and the outlawed spiritual movement Falun Gong - is either taboo or made to conform to the party line. Scandals involving political leaders and mass protests often never appear in the mainland media. Historical memories are also selective. Sensitive anniversaries - including those for the 1989 massacre, the start of the Cultural Revolution or the anti-rightist campaign - go largely unmentioned.

According to the Committee to Protect Journalists, however, China jails more reporters than any other country. Reporters Without Borders ranks the country among the 10 worst in the world for press freedom.

The overall picture is more complex. The proliferation of mobile phones and the internet has made it harder to block news. The government has also relaxed controls on coverage of environmental disasters, social inequality and mine accidents. Earlier this year, it eased restrictions on the movements of foreign correspodents.

And the censors occasionally slip up. In 2004, the website of the People's Daily - the mouthpiece of the communist party - carried a story referring to the "violent crackdown in Beijing's Tiananmen Square" on the pro-democracy student movement. The lines were at the end of an obituary that had been cut and paste from an article in the Hong Kong media. They were removed after being spotted four hours later.

In 1991, the People's Daily unwittingly propagated subliminal sedition by publishing a poem, which - if read diagonally - contained a coded call for then prime minister Li Peng to step down. Li - dubbed the "Butcher of Beijing" by his enemies - is among the most reviled figures in recent Chinese history because he ordered martial law in 1989.