They were among 45 opposition figures, including some of Hong Kong's best-known democracy activists, who were sentenced in November over a 2020 informal primary election that authorities deemed a subversive plot.

Critics including the United States, Britain and the European Union said the case showed how a Beijing-imposed national security law has eroded freedoms and quashed peaceful opposition in Hong Kong.

Ex-lawmakers "Long Hair" Leung Kwok-hung, Lam Cheuk-ting, Helena Wong and Raymond Chan are among those contesting their convictions and sentences in hearings that are scheduled to last 10 days.

Owen Chow, a 28-year-old activist who was sentenced to seven years and nine months in jail -- the harshest penalty among the 13 -- has also lodged an appeal.

Some of the appellants have already spent more than four years behind bars.

Amnesty International's China director Sarah Brooks said the appeal will be a "pivotal test" for free expression in the Chinese finance hub.

"Only by overturning these convictions can Hong Kong's courts begin to restore the city's global standing as a place where rights are respected and where people are allowed to peacefully express their views without fear of arrest," Brooks said.

- Morning raids -

Prosecutors will concurrently challenge on Monday the lower court's acquittal of lawyer Lawrence Lau, one of two people found not guilty from an original group of 47 accused.

Activist Tam Tak-chi, who pleaded guilty in the subversion case, had also indicated he would appeal against his sentence but withdrew.

Beijing has remoulded Hong Kong in its authoritarian image after imposing a sweeping national security law in 2020 following months of huge, and sometimes violent, pro-democracy demonstrations.

Authorities arrested figures from a broad cross-section of the city's opposition in morning raids in 2021, a group later dubbed the "Hong Kong 47".

The group, aged between 27 and 69, included democratically elected lawmakers and district councillors, as well as unionists, academics and others with political stances ranging from modest reformists to radical localists.

They were accused of organising or taking part in an unofficial primary election, which aimed to improve the chances of pro-democracy parties of winning a majority in the legislature.

The activists had hoped to force the government to accede to demands such as universal suffrage by threatening to indiscriminately veto the budget.

Three senior judges handpicked by the government to try security cases said the plan would have caused a "constitutional crisis".

Beijing and Hong Kong officials have defended the national security law as being necessary to restore order following the 2019 protests.

Opposition party the League of Social Democrats -- co-founded by Leung -- announced its disbandment last month, citing "immense political pressure".

Eight of the jailed campaigners, including journalist and lawmaker Claudia Mo and LGBTQ activist Jimmy Sham, have been released in recent weeks after completing their sentences.