The Chinese city used to hold annual candlelight vigils to mark Beijing's deadly crackdown on demonstrators in and around Tiananmen Square on June 4, 1989, but those events have been banned in recent years.

Lee Cheuk-yan and Chow Hang-tung, who organised vigils as leaders of the now-defunct Hong Kong Alliance, are standing trial for "incitement to subversion", which carries a maximum penalty of 10 years in jail.

Seated in the dock, a grim-faced Lee wiped his eyes on Monday as large screens in the courtroom showed him calling for a "democratic China" in 1996, a year before Hong Kong's handover to Chinese rule.

Fiery rhetoric denouncing the Chinese Communist Party was once seen as unremarkable in Hong Kong, but it has all but vanished after the imposition of a national security law in 2020.

Prosecutor Ned Lai denied that the proceedings were a "political trial".

Lai's team said they would focus on the Alliance's repeated calls to "end one-party rule" in China, a core tenet of the group since its founding in 1989, arguing that it amounted to subverting the state.

Lee, 68, and Chow, 41, maintained their political stance even after the security law took effect, Lai added.

A video clip from 2020 showed Lee saying, "We hold on to our principle and we would not retreat... It's very important that we believe in democracy and we can practise our belief."

Chow, who is a barrister, represented herself and often smiled at supporters in the public gallery.

"We will still be in Hong Kong, to defend the truth about June 4 and continue to resist dictatorship," Chow was heard saying in a clip from May 2021.

Dozens of other clips were played, showing the defendants speaking at vigils, protests and press interviews over the years.

Hong Kong authorities say that the national security law has no retroactive effect.

Chow and Lee, who are standing trial after pleading not guilty, have been behind bars since 2021.

Chow applied for the court to hear expert evidence from Taiwanese sociologist Ho Ming-sho but was rejected by the three-judge panel.

A third defendant in the case, 74-year-old Albert Ho, pleaded guilty last week and was excused from the trial, which was scheduled to last 75 days.

The trial was condemned last week by Amnesty International and Human Rights Watch as an attempt at "rewriting history".