The proceedings are expected to last more than four months, and the defendants face sentences of up to life imprisonment if convicted.

Authorities have accused the 47 of trying to topple Hong Kong's China-approved government.

The defendants say they have been targeted for normal opposition politics, with observers saying the trial illustrates how little room there is left to criticise China's rule since huge pro-democracy protests in 2019 were stamped out.

Those on trial represent a cross-section of Hong Kong's opposition -- from prominent legal scholar Benny Tai, to former lawmakers such as Claudia Mo, Au Nok-hin and Leung Kwok-hung, to younger democracy activists such as Joshua Wong and Lester Shum.

The group were jointly charged in March 2021 with "conspiracy to commit subversion" for organising an unofficial primary a year earlier to select opposition candidates for a legislature election.

Their stated aim was to win a majority in the city's partially elected legislature, which would allow the bloc to veto budgets and potentially force the resignation of Hong Kong's leader.

That election was ultimately scrapped by authorities and Beijing brought in a new political system that strictly vetted who could stand for office.

The group was charged en masse under the national security law that China imposed on Hong Kong in 2020.

China says the law was needed to curb political unrest, but rights groups and Hong Kong opposition figures say an ensuing crackdown has all but ended the city's autonomy and political freedoms.

Dennis Kwok, a former opposition lawmaker who now lives in the United States, described the prosecution of the 47 as "a complete farce".

"Subversion is a crime that used to require someone who threatened to use violence... to overturn the regime," Kwok told AFP.

"It doesn't include people who simply run for office and pledge to use their public office to force the government to respond to the demands of the people they represent."

Prosecutors and government supporters see the opposition primary differently.

"I would assume if your intent is to bring down the government, then that must be unlawful," Ronny Tong, a veteran lawyer who sits on Hong Kong's cabinet, told AFP.

While Hong Kong has never been a democracy, its system of governance allowed, for a time, far more freedom of expression than in mainland China.

The national security law has since transformed the city's political landscape as well as its common law legal traditions, with each arrest and prosecution setting new precedents.

Protesting and challenging authorities is now fraught with legal risk.

The law also empowered China's security apparatus to operate openly in the city, weaving in a new legal system that made Hong Kong courts more closely resemble the mainland's.

Most of the defendants -- 34 out of 47 -- have been jailed for almost two years. The few granted bail have to abide by strict conditions, including speech restrictions.

Judges who sit on national security cases are handpicked by the city's leader and there has not yet been a trial in front of a jury.

In December, Beijing said Hong Kong's leader could also bar foreign lawyers from taking part in national security trials.

Legal and political analysts are watching the trial of the 47 activists closely.

"This particular charge and this case will send quite a strong signal that any challenge to the authority of the current regime will be taken seriously," Ming-sung Kuo, a legal scholar at Warwick University, told AFP.

Eric Lai, a fellow of Georgetown University's Center for Asian Law, said Hong Kongers will be paying close attention to "how the prosecution defines an ordinary civil society event as a criminal act".

Sixteen of the 47 have pleaded not guilty, a stance that, if they are convicted, could lead to longer sentences.

At least three will testify against their peers as prosecution witnesses, the court has been told.