Chinese authorities have released the last prisoner convicted of "counter-revolutionary" charges in connection with the 1989 Tiananmen Square demonstrations.

Jiang Yaqun, 73, who has Alzheimer's disease, was released from a Beijing prison without a home or family to go to, or a source of income, the San Francisco-based Dui Hua Foundation said on Thursday, citing an undated notice posted on the website of Jingshan neighbourhood council.

Tuesday will mark the 24th anniversary of the day that Chinese soldiers quashed the largely student-led pro-democracy movement by opening fire on demonstrators, killing hundreds of people.

Fearing a crisis of legitimacy, China's ruling Communist party has since forbidden open discussion of the events of 4 June 1989 in classrooms, the media and online.

"For me the significance of this case is that this was a guy who, despite his age and his poor health and, I believe, his poor psychological condition, spent a considerable amount of time in prison," said Joshua Rosenzweig, a researcher at the Chinese University of Hong Kong and a former manager at Dui Hua. "And most likely simply because of the sensitivity of the incident which he got caught up in."

Counter-revolutionary charges were removed from Chinese criminal law in 1997, but according to Dui Hua, a "handful" of people are still incarcerated in Beijing on other charges related to the protests, such as arson and assault. Most of them related to "attempts to prevent the military from entering and spreading out through Beijing", Rosenzweig said.

In July 1990, the Beijing high people's court sentenced Jiang, the in his mid-40s, to death with a two-year reprieve, Dui Hua said. While the details of his case remain unknown, his sentence had been reduced five times. He was diagnosed with "mild mental retardation" and transferred to Beijing's Yanqing prison, which has a ward for the infirm, in 1993.

Dui Hua believe Jiang was released some time in October or November last year, based on the Jingshan website notice.

On Friday, the pro-democracy group Tiananmen Mothers published an essay excoriating the Communist party's leadership for their refusal to confront past mistakes. The organisation, comprised of activists whose friends and family members died in the crackdown, has been sending open letters, petitions and dialogue proposals to China's leadership since 1995. It has yet to receive a response.

"During these long 24 years, we Tiananmen Mothers have suffered profoundly," said the essay, according to a translation by the New York-based NGO Human Rights in China. "We have moaned in hell-like darkness, struggled in tears which nearly dried up."

The essay, which has 123 signatories, also criticised China's newly anointed president, Xi Jinping, for his refusal to reassess party history beginning with the purges and famine under Mao Zedong. "It appears that Mr Xi Jinping categorically does not care about the tens of millions of lives of his fellow countrymen," it said.