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

But the vigils have been banned since 2020, when Beijing imposed a national security law on the financial hub following huge, sometimes violent pro-democracy protests the year before.

Three leaders of the now-disbanded vigil organiser group -- known as the Hong Kong Alliance -- face charges of "incitement to subversion" under the security law, which carry up to 10 years in prison.

The vigil organisers stood accused of using slogans and speeches to incite others to use "unlawful means" to end the Chinese Communist Party's leadership, according to a prosecution document read out on Monday.

Defendant Chow Hang-tung, who is also a barrister, argued that the indictment should be quashed as prosecutors had not made clear the meaning of "unlawful means".

Judge Alex Lee said prosecutors cast a "very wide" net in their interpretation of the offence, but that did not mean the trial could not proceed.

Prosecutors admitted that the activists did not offer "concrete plans" while inciting people to overthrow Beijing, he added.

Lee, as part of a three-judge panel, ruled that the trial will start on the morning of January 22.

Chow and another defendant have already spent more than four years behind bars.

She was arrested the morning of June 4, 2021, when her articles appearing on social media and in a newspaper called on residents to "light candles to seek justice for the dead".