Domestically, Beijing heavily censors any mention of the topic, and the Hong Kong trial has been condemned by human rights groups as an attempt to "rewrite history".

Here is what we know:

- What were the Tiananmen vigils? -

On June 4, 1989, the Chinese government sent troops and tanks to break up peaceful protests on and around Beijing's Tiananmen Square, crushing a weeks-long wave of demonstrations calling for political change and curbs on official corruption.

Hundreds, by some estimates more than 1,000, were killed in the crackdown.

The news sent shockwaves through Hong Kong's population, fuelling anxieties in the former British colony ahead of its handover to China in 1997.

Many of the city's residents cite Tiananmen as their political awakening.

The Hong Kong Alliance was a group founded in May 1989 to support the demonstrators, and it subsequently became the driving force seeking redress on their behalf.

The group's key tenets included "building a democratic China" and "ending one-party rule", referring to the Chinese Communist Party.

It organised vigils in Hong Kong's Victoria Park every June 4, with thousands of attendees turning the grounds into a sea of candlelight.

The Alliance also operated a museum dedicated to the crackdown and held events such as a yearly washing of the Pillar of Shame, a statue to mourn victims.

"What Hong Kong Alliance did... was of utmost importance to freedom-loving Chinese (people) who refuse to forget," US-based Tiananmen survivor Zhou Fengsuo told AFP.

Alliance leaders often described themselves as Chinese patriots, which drew flak from Hong Kong's more radical pro-democracy factions in the 2010s.

- How was remembrance suppressed? -

Beijing imposed a national security law on Hong Kong in 2020 after huge, sometimes violent pro-democracy protests in the Chinese finance hub the previous year.

The vigil was banned on public health grounds that year because of the pandemic, and has not returned since.

In September 2021, Hong Kong authorities arrested Alliance leaders and raided its museum.

The group voted to disband that same month.

The Pillar of Shame was dismantled and removed from its longtime perch at the University of Hong Kong in late 2021, and other local universities followed suit with similar monuments.

Books related to the crackdown were removed from public library shelves around the same time, according to Hong Kong media.

Over the years, authorities have arrested multiple people for Tiananmen-related social media posts.

Police were deployed in force around Victoria Park on recent anniversaries, with officers searching and detaining some mourners.

Zhou, the Tiananmen survivor, said diaspora communities in places like Britain and Canada are carrying the torch of remembrance.

"Hong Kongers all over the world are still commemorating... and still remember the candlelight of Victoria Park," he told AFP.

- Who is facing trial? -

Lee Cheuk-yan, Albert Ho and Chow Hang-tung are among the former leaders of the Alliance.

They were charged with "incitement to subversion" in 2021.

Ho, 74, pleaded guilty on Thursday. He was a lawyer-turned-politician and led Hong Kong's now-dissolved Democratic Party.

Ho was granted bail in 2022 for medical treatment but was put back behind bars less than a year later.

Lee, 68, has a trade union background and led the now-defunct Labour Party. Lee and Ho each served as pro-democracy lawmakers for more than two decades.

Chow, 40, is a Cambridge-educated barrister and is representing herself in Thursday's trial.

She joined the Alliance in 2010 as a volunteer and is part of a generation who came of age after Hong Kong's return to Chinese rule.