Hong Kong used to be the only place in China where people could publicly mourn Beijing's deadly crackdown on pro-democracy demonstrators in Tiananmen Square on June 4, 1989.

But Beijing imposed a national security law on the former British colony in 2020 following huge and sometimes violent pro-democracy protests the year before.

The vigil has been banned and its organiser, known as the Hong Kong Alliance, shut down after authorities arrested three of its leaders.

The trio and the Alliance itself are charged with "incitement to subversion" under the security law, with the no-jury trial expected to last around 75 days.

Defendants Chow Hang-tung and Lee Cheuk-yan have been behind bars since 2021 and will contest their charges on Thursday. The third defendant Albert Ho is set to plead guilty.

The Alliance had repeatedly called for the "end of one-party rule" in China, which prosecutors said amounted to subverting state power, according to a case document published on Wednesday.

The prosecution will rely on company records, online material, clips of public speeches and evidence seized from the now-defunct Tiananmen museum operated by the group.

US-based Tiananmen survivor Zhou Fengsuo told AFP he was "deeply concerned" for the trio and that the Hong Kong vigils used to be "a source of hope, justice (and) comfort".

"They are just defending the historical truth... they represent the conscience of a free Hong Kong that was destroyed," he said.

Authorities last year barred overseas witnesses from testifying remotely in national security cases.

Amnesty International said on Thursday that the trial was a further move by Hong Kong to silence dissent.

"This case is not about national security -- it is about rewriting history and punishing those who refuse to forget the victims of the Tiananmen crackdown," said Amnesty's deputy regional director Sarah Brooks.

Hong Kong authorities say the prosecutions are safeguarding human rights and based on evidence.

- 30 years of vigils -

The Hong Kong Alliance was founded in May 1989 to support students holding democracy and anti-corruption rallies in Beijing.

The following month, the government sent tanks and soldiers to crush the movement in Tiananmen Square, a decision it has since heavily censored and removed from public record in mainland China.

Over the next three decades, the Alliance called on Beijing to accept responsibility, free dissidents and embrace democratic reform.

The group organised annual candlelight vigils in Hong Kong's Victoria Park every June 4, which routinely drew thousands of attendees and became a weathervane for the city's political freedoms.

In 2021, the Alliance refused to turn over details on group members and finances to Hong Kong's national security police -- a decision that sparked a criminal prosecution.

Tang Ngok-kwan, a former Alliance member involved in that earlier case, told AFP that he hoped the upcoming trial will be a chance to revisit history.

"By having a venue to debate China's constitutional development, I hope the case will have an impact on the future," Tang said.

The Tiananmen trial is the latest high-profile national security case in Hong Kong, following the conviction of pro-democracy media tycoon Jimmy Lai last month, which drew international condemnation.

Lai was found guilty of conspiring to commit foreign collusion and judges have yet to determine his sentence.

Chief Justice Andrew Cheung responded to the criticisms on Monday, saying that judges deal "only with the law and the evidence, not with any underlying matters of politics".

US President Donald Trump has reportedly asked Chinese leader Xi Jinping to release Lai.