Shiu Ka-chun, a social worker who represented his profession in Hong Kong's legislature, died last month aged 55 after earlier revealing he was diagnosed with stomach cancer.

Watched by dozens of plainclothes police, people queued to enter the Methodist International Church on Saturday morning to pay their last respects, followed by a closed-door memorial service.

"(Shiu) has a unique place in history... When I think of prisoners' rights, I think of him," said political scientist Ivan Choy outside the church.

A social worker and university lecturer, Shiu rose to prominence during Hong Kong's 2014 Umbrella Movement -- a civil disobedience campaign to push for genuine elections -- and was later jailed for it.

During the 163 days he spent behind bars, Shiu clashed with guards over prisoners' welfare and he championed the cause after his release.

Wall-fare, a support group started by Shiu, was credited with helping families navigate the maze of prison bureaucracy -- a need that became more urgent as thousands were imprisoned over Hong Kong's 2019 protests.

A young woman surnamed Wong said she started paying attention to prisoners' rights after joining Shiu's pen pal programme that connected her to people behind bars.

"He was the reason I became interested in prisoners' rights... he made me want to do more, even though I might not be able to change much," Wong told AFP.

Shiu wanted to make prison life more bearable and arranged to mail books to prisoners, said a novelist surnamed Chiang who collaborated with the activist.

"He devoted himself because of his Christian faith and his commitment to society. That's very admirable," she said.

Wall-fare shut down in 2021 after Hong Kong's security chief accused it of endangering national security.

Activist Dickson Chau from the League of Social Democrats hailed Shiu as a "pioneer" within the legislature who tried to change the system.

Shiu was a prolific writer and his books -- along with keepsakes from his lawmaker and lecturer days -- were displayed at Saturday's memorial.

Veterans from Hong Kong's pro-democracy camp, including Cardinal Joseph Zen and barrister Margaret Ng, were among those who paid tribute.

In his final months, Shiu wrote about his struggle with cancer and said he was "losing weight, not faith".

"Joy and suffering are two extremes and both are necessary to life."