Once legal in China-ruled Hong Kong, such public commemorations were hailed as a symbol of the city's relative freedom compared to mainland China.
The events on June 4, 1989, when Chinese troops opened fire to end the student-led protests, are not publicly discussed in China, which treats the date as taboo and allows no public remembrance.
Several June 4 monuments, such as the "pillar of shame", a memorial for those killed in the crackdown that depicted dozens of torn and twisted bodies were also removed from three Hong Kong universities.
Blocked in 2020 over COVID-19 curbs, the commemorations have never resumed since China imposed a tough national security law that year.
Under that law, Lee Cheuk-yan, 68, Albert Ho, 74, and Chow Hang-tung, 40, the three former leaders of the now-disbanded group, now face charges of "inciting subversion of state power" that carry punishments of up to 10 years in jail.
The trial on Thursday is among the last of several such major cases, with Chow, the former vice chair of the group, the Hong Kong Alliance in Support of Patriotic Democratic Movements of China, held on remand for more than 1500 days after being denied bail.
In an opening statement, prosecutors said the case centred on whether the alliance's publicly stated goal of "ending one-party rule" constituted illegally inciting others to carry out acts aimed at subverting state power.
The other key focus of the case was whether such acts amounted to "overthrowing or undermining" China's system of government.
Rights groups and some foreign governments have criticised such national security cases against prominent democrats as a weaponisation of the rule of law to silence dissent.
Beijing, however, says the security law was necessary to restore order after sometimes violent protests rocked the Asian financial hub for months in 2019.
Detained since September 2021, Chow, a Cambridge-educated barrister, is one of the few democratic campaigners still speaking out against the Chinese Communist Party's crackdown.
She has represented herself in court and challenged prison rules, such as one barring female inmates from wearing shorts in summer.
"The state can lock up people but not their thinking, just as it can lock up facts but not alter truth," she told Reuters in an interview.
Last November, the High Court rejected Chow's bid to terminate the trial, and barred her from calling overseas witnesses to testify virtually in 2024.
In another judgment on Wednesday, the judges said the court would adjudicate on the basis of evidence and legal principles and "will not allow trials to become a tool for political repression ... or an abuse of judicial procedures", as Chow claimed.
