Authorities used a Beijing-imposed national security law to charge 47 people -- including some of Hong Kong's best-known democracy advocates -- with subversion after they held a primary election in 2020 in a bid to win a legislative majority.

Former Hong Kong opposition lawmaker Claudia Mo, 68, left the Lo Wu Correctional Institution just before sunrise on Tuesday after completing her sentence, according to police.

Three co-defendants -- Kwok Ka-ki, Jeremy Tam and Gary Fan -- also finished serving their sentences and left Hong Kong's Stanley Prison and Shek Pik Prison at around the same time.

All four people were taken out of prison in cars with curtains drawn, with police at the scene confirming that the democrats were in those vehicles.

The four opposition figures pleaded guilty and were each sentenced to four years and two months behind bars in November.

The sentences have been condemned by Western governments and rights groups, with the United States calling the penalties "unjust" and the United Nations right office expressing what it called grave concern.

The four, who had been in custody since March 2021, received the lightest penalty among the defendants and were released on Tuesday taking into account the time they served before going on trial.

Beijing imposed a sweeping national security law on Hong Kong in 2020 following huge and sometimes violent pro-democracy protests in the finance hub.

Under that law, the 47 Hong Kong democrats were charged with conspiring to subvert state power via a multi-step plan that began with an informal poll within their camp and ending with an indiscriminate veto of the government budget.

The landmark case involved figures across Hong Kong's once-diverse political spectrum -- including elected lawmakers, district councillors, unionists and academics with views ranging from moderate to radical.

Mo worked as an Agence France-Presse journalist and previously cited her experience covering the 1989 Tiananmen Square crackdown as pivotal for her political awakening.

She helped found the now-shuttered Civic Party in 2006 and won a legislative seat in 2012, but later quit the party to take a more localist stance.

Kwok, 63, and Tam, 49, were also former Civic Party lawmakers. Before entering politics, Kwok worked as a urology doctor and Tam as an airline pilot.

Fan, 58, was a co-founder of Neo Democrats, a party that advocated for electoral reform and pushed back against China's political and cultural influence on Hong Kong in the 2010s.

Each of the four defendants had their prison terms trimmed due to their guilty pleas, with an additional six-month reduction on account of "past public service and ignorance of the law".

Hong Kong tightened its rules last year so that prisoners convicted of serious national security crimes would not be released early for good behaviour.

The heaviest penalty in the case -- a 10-year jail sentence -- was imposed on legal academic Benny Tai, whom prosecutors described as the "mastermind".

The city has arrested 322 people for national security crimes and convicted 163 of them as of the start of this month.

Hong Kong last year enacted a homegrown national security law on top of the Beijing-imposed law, an arrangement officials say is needed to restore order.