Jimmy Sham was one of 45 opposition figures jailed for subversion after holding an informal election primary in 2020, in what was the city's largest national security case.

The 37-year-old was one of four people released on Friday after serving their sentences -- the second batch from the 45 to be freed in recent months.

"It's the first day, so I'm still trying to figure out what I can and should do in the future, and where the red lines are," Sham told local media reporters.

Civil and political rights in the city have been curtailed since Beijing imposed a sweeping national security law on Hong Kong in 2020, after enormous and sometimes violent pro-democracy protests the year before.

Sham was the convenor of the now-defunct Civil Human Rights Front, one of the marches' largest organising groups.

- 'Out of touch' -

A well-known LGBTQ activist, Sham wore a locally designed T-shirt featuring a parade of cartoon creatures waving a rainbow flag as he was released.

While he was in jail, judges ruled on a case he had brought asking the city's top court to strike down Hong Kong's same-sex marriage ban.

"(The case) was a blessing to me. I was in a mild depression... It made me feel as if something could change, and it made me feel empowered," he said Friday.

Judges rejected same-sex marriage but ordered the government to set up an "alternative legal framework" to protect same-sex couples' rights.

"It's a shame that the case was lost, but having a framework that can be established (in the future) has been a big change," Sham said.

But after four years and three months in jail, he said he had a lot to catch up on and learn from others.

"I think I am out of touch with society," he said, adding he wanted to spend time with his family before anything else.

He walked free along with three former district councillors, Roy Tam, Henry Wong, and Kinda Li.

Their release follows that of ex-legislators Claudia Mo, Jeremy Tam, Kwok Ka-ki and Gary Fan, also part of the 45, in late April.

Sham said he had "mixed feelings" about his freedom.

"I've experienced a lot of unhappiness and powerlessness, so I hope we can all pull ourselves together, and try to get on with our lives anyway," he said.

"I won't leave Hong Kong at this moment, as many who are meaningful to me are still staying here," he added.