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 organised by democracy supporters in 2020.

More than 100 people queued for seats in the court's public gallery with dozens of police on standby and a bomb disposal vehicle deployed.

"There's no crime to answer. It is not a crime to act against a totalitarian regime," defendant and former lawmaker Leung Kwok-hung, known by the nickname "Long Hair", told the court.

Judge Andrew Chan responded that the hearing was a "solemn occasion" and asked for respect from the defendants and members of the public.

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

"Running for the LegCo election is what kind of illegal means, what kind of violent threat?" Chan Po-ying, chairwoman of League of Social Democrats and Leung's wife said outside the court.

A small group of protesters also gathered outside the court this morning.

"Crackdown is shameless," read a banner carried by two people, adding: "Immediately release all political prisoners."

Another protester was seen raising a fist in solidarity as a large number of police stood nearby.

The trial is expected to last 90 days, with three defendants expected to testify against the others, prosecutors say. The defendants face sentences of up to life in jail if convicted.

Those who have pleaded not guilty include former journalist Gwyneth Ho, activist Owen Chow and labour unionist Winnie Yu.

All who pleaded guilty, including former law professor Benny Tai and prominent activist Joshua Wong, will be sentenced after the trial.

The then British colony was returned to China in 1997 under a "one country, two systems" formula meant to guarantee its freedoms and an independent legal system for 50 years.

China denies interfering with the city's way of life but some Hong Kong residents over the years criticised what they saw as the erosion of freedoms by an increasingly assertive Beijing.

Beijing imposed a national security law on the city in 2020 after months of at times violent pro-democracy protests.

Western governments have criticised the law, which punishes subversion, collusion with foreign forces and terrorism with up to life in prison, as a tool to crush dissent.

Chinese and Hong Kong authorities say the law has brought stability to the Asian financial hub.

Since the law was imposed, more than 230 people have been arrested, including newspaper editors following police raids on media outlets, while labour unions and civil society groups have disbanded.

Thirteen of the 47 arrested were granted bail in 2021, while the other 34 - including 10 who pleaded not guilty - have been in pre-trial custody.

The case has drawn international criticism, as government prosecutors repeatedly asked for more time to prepare legal documents and gather evidence.

Among a number of departures from established common law procedures, Secretary for Justice Paul Lam declined to let the defendants face a jury trial. The case is being heard by three High Court judges designated under the national security law: Andrew Chan, Alex Lee and Johnny Chan.