Ex-legislators Claudia Mo, Jeremy Tam, Kwok Ka-ki and Gary Fan were part of a group of 47 public figures -- including some of Hong Kong's best-known democracy advocates -- were charged with subversion in 2021 for holding an informal primary election.

The case fell under a national security law imposed on the city by Beijng and drew international condemnation and warnings about Hong Kong's declining freedoms and tolerance of dissent.

Mo, Tam, Kwok and Fan had been kept in custody since 2021 and were each sentenced to four years and two months behind bars after they pleaded guilty.

All four were taken out of prison just before sunrise on Tuesday in cars with curtains drawn.

Speaking outside his home, Mo's husband Philip Bowring said the ex-lawmaker was resting and not in a position to speak to the media.

"She's well and she's in good spirits... We look forward to being together again," Bowring said at his flat, with a "Welcome home mum" banner visible in the dining room.

"We'll be here for a while and getting used to living in Hong Kong again and then probably we'll go to England at some point to see our grandchildren," he added.

Fan told local media he was on his way to reunite with his family and thanked Hongkongers for their concern.

- 'Unjust' -

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

Authorities said an informal primary election held by the former lawmakers which aimed to win a legislative majority, with the ultimate goal of indiscriminately vetoing the government budget, amounted to a conspiracy to subvert state power.

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.

The prison sentences, delivered in November last year, were condemned by Western governments and rights groups.

Mo previously worked as an AFP journalist and cited her experience covering Beijing's bloody 1989 Tiananmen Square crackdown as pivotal in 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 campaign on a platform emphasising Hong Kong's distinctive identity from mainland China.

Kwok, 63, and Tam, 49, were also former Civic Party lawmakers. Before entering politics, Kwok worked as a 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.

- Tightening grip -

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 could not be released early for good behaviour.

The four ex-lawmakers who returned home on Tuesday received the lightest penalty among the defendants and were released taking into account the time they served before trial.

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

The court is set to hear appeals launched by 14 of the convicted defendants in July.

Hong Kong has arrested 322 people for national security crimes. It has convicted 163 of them as of the start of this month.

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