Claudia Mo, 68, was among the 45 Hong Kong opposition figures imprisoned in the city's largest national security case, after they held an informal election in 2020 that authorities deemed a "conspiracy to subvert the state power".

Mo and ex-lawmakers Jeremy Tam, Kwok Ka-ki and Gary Fan were each released on Tuesday after completing a jail term of four years and two months -- the first batch of defendants to regain their freedom.

"Many thanks for all the concern and care expressed upon my release," Mo wrote on Facebook on Friday.

"Prison life was surreal, almost Kafka-esque to start with," she added.

"But I didn't suffer the two major incarceration traumas, loneliness and boredom, thanks to the social arrangements inside."

Mo posted a photo "taken (a) couple hours after getting home from prison" that showed her in front of a banner reading "Welcome home mum".

The ex-lawmaker thanked her family and friends, including Cardinal Joseph Zen, Reporters Without Borders and the now-shuttered minority rights group Hong Kong Unison.

"My thoughts are with my co-defendants who remain in custody," she added.

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

The court last year jailed 45 pro-democracy figures under that law, including some of Hong Kong's best-known activists and figures from across the city's once-diverse political spectrum.

Mo and the three other Hong Kong democrats released on Tuesday had pleaded guilty, which led to a reduced sentence.

She said in her Friday Facebook post that she had read more than 300 books, mainly novels, and improved her French while behind bars.

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.

On Tuesday, she was taken out of prison just before sunrise in a convoy of vehicles with curtains drawn.

Hong Kong authorities said it made "appropriate arrangements" based on factors including prison security and inmates' privacy and safety.

Shortly after Mo returned home, her husband Philip Bowring said she was resting and not in a position to speak to media.