Sixteen Hong Kong pro-democracy figures face trial on Monday, more than two years after their arrest, in what some observers say is a landmark case for the city's judicial independence under a national security law imposed by Beijing.

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 in 2020.

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

Western governments have criticised the 2020 national security law as a tool to crush dissent in the former British colony. Chinese and Hong Kong authorities say the law, which punishes subversion, collusion with foreign forces, and terrorism with up to life in prison, has brought stability to the Asian financial hub after huge pro-democracy protests in 2019.

Prosecutors have described the primary election - held to select the strongest candidates to contest an upcoming legislative election - as a "vicious plot" to subvert the government and to wreak "mutual destruction" on the city by taking control of the city's parliament.

The lengthy, high-profile case has drawn international criticism, as government prosecutors repeatedly requested more time to prepare legal documents and gather more evidence.

"This trial is not simply a trial against the 47 opposition leaders but also a trial for the population who has been supporting the pro-democracy movement for decades," Eric Lai, a fellow at Georgetown Centre for Asian Law in Washington, told Reuters.

The trial is expected to last 90 days, with three defendants expected to testify against the others, prosecutors say.

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

"The actual people who need to go on trial are absolutely not us," Chow wrote on his Facebook page in September. "We're not guilty at all."

The 31 who pleaded guilty, including former law professor Benny Tai and activist Joshua Wong, will be sentenced after the trial.

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

Pretrial proceedings were largely kept out of the public eye until judge Lee agreed to lift reporting restrictions in August.

- Reuters