The pro-democracy advocates were among 47 activists charged in 2021 with conspiracy to commit subversion for their involvement in an unofficial primary election.
The mass prosecution involving some of the best-known activists crushed much of the city's once-thriving pro-democracy movement that reached a height with massive anti-government protests in 2019.
Forty-five of the defendants were sentenced to between four years and 10 years in 2024, with their punishments drawing criticism from foreign governments and rights groups.
Eleven activists who appealed their convictions lost their bids. They included former lawmakers Leung Kwok-hung, Lam Cheuk-ting, Raymond Chan and Helena Wong.
All appeals over sentences were also dismissed by the Court of Appeal on Monday.
Lawrence Lau, a pro-democracy former district councillor, was one of two activists acquitted in the case. Judges upheld his acquittal following an appeal from the prosecution.
Riding on the 2019 protests, the pro-democracy camp had been looking to make gains in the 2020 legislative election. The unofficial primary was meant to shortlist pro-democracy candidates for the official election.
The camp hoped to secure a majority in the legislature to press for protesters' demands, which included greater police accountability and democratic elections for the city's leaders.
During the trial, prosecutors said the activists aimed to paralyse Hong Kong's government and force the city's leader to resign.
The judges said in their verdict in 2024 that the activists' plans to effect change through the unofficial primary would have undermined the government's authority and created a constitutional crisis.
Critics said the activists' convictions illustrated how authorities crushed dissent following the 2019 protests. The Beijing and Hong Kong governments insist the national security law was necessary for the city's stability.
The case involved democracy advocates across the spectrum, including legal scholar Benny Tai, who got a 10-year prison term, and former student leader Joshua Wong, whose sentence was four years and eight months.
Nearly 20 activists in the case have been released from prison over the past year.
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