The defendants are those who pleaded not guilty out of 47 arrested in a dawn raid in January 2021 and charged with conspiracy to commit subversion for participating in an unofficial primary election organized by democracy supporters in 2020.

Among the 16 are former journalist Gwyneth Ho Kwai-lam, former lawmaker Lam Cheuk-ting and "Long Hair" Leung Kwok-hung, activist Owen Chow Ka-shing and labor unionist Winnie Yu Wai-ming.

Prosecutors have described the primary election, held to select the strongest candidates to contest an election for the Legislative Council, as a "vicious plot" to subvert the government and to wreak "mutual destruction" on it by taking control of the LegCo.

The trial is expected to last 90 days, with four of the 47 activists agreeing to turn prosecution witnesses and testify against the 16, prosecutor Jonathan Man told the court.

Man identified the four as former legislator Au Nok Hin, former district councilors Andrew Chiu Ka-yin and Ben Chung Kam-lun, and businessman Mike Lam King-nam.

In previous proceedings, 18 activists had indicated they intended to plead not guilty. But two of them — former district councilor Ng Kin-wai and Mike Lam King-nam — later changed their minds, joining the other 29 activists who plan to admit the charges.

The trial is being heard by three High Court judges designated under the national security law: Andrew Chan, Alex Lee, and Johnny Chan.

Most of the 47 activists who were charged with conspiracy to commit subversion have been detained without bail for nearly two years, former lawmakers Raymond Chan Chi-chuen and Helena Wong Pik-wan, who appeared in court on Monday, were among the minority who were granted bail based on strict conditions.

Those who pleaded guilty, including former law professor Benny Tai Yiu-ting and young activist Joshua Wong Chi-fung, will be sentenced after the trial.