Joshua Wong, along with Nathan Law and Alex Chow, was last year found guilty
of taking part in an unlawful rally in connection to their involvement in the 2014 pro-democracy street protests known as the "Umbrella Movement."
The protests shut down large parts of the city for months, and saw Joshua Wong feature on the front page of TIME magazine.
Wong, 20, and Law, 24, were given community service sentences and Chow, 26, a suspended three-month jail term.
The case appeared closed, but this month prosecutors requested the city's Court of Appeal review their sentences, urging judges to hand down jail time as a deterrent for future protesters.
Under Hong Kong's Public Order Ordinance, the court could jail the three for up to five years.
Speaking to reporters Wednesday, Wong said he expected to be jailed, pointing to a decision by the court this week to increase punishments for 13 activists who attempted to force their way into the city's parliament in 2014.
The defendants, all of whom had already carried out community service orders they were given by a lower court, were jailed for between eight and 13 months, according to local broadcaster RTHK
"Following the sentence review ... I expect to face immediate imprisonment (on Thursday)," Wong said. "However, I still hope to encourage Hong Kong people to keep on fighting for democracy."
Pro-democracy lawmaker Eddie Chu accused the government Wednesday of engaging in "political persecution" by appealing the cases.
"(They are) trying to raise the bar of political dissent to long term imprisonment in order to deter future actions by the citizens of Hong Kong," Chu told RTHK
In a statement released Thursday, Hong Kong's Department of Justice said the three defendants broke the law with "disorderly and intimidating behavior."
"There cannot be any suggestion that the court was not acting independently and fairly in convicting the defendants," the statement said, adding "there is absolutely no basis to imply any political motive on the part of the Department of Justice in this case."
Nathan Law warned followers on Instagram
Wednesday that it might be his last post "for a while."
The city's youngest-ever elected lawmaker, Law was last month removed from office
after a court ruled he and three others did not take their oaths of office properly, in a case that has been widely denounced
as an attempt to eject democratically elected opposition lawmakers.
US-based Human Rights Watch (HRW) said Wednesday there has been a "surge in politically motivated prosecutions against Hong Kong's pro-democracy leaders" since the end of the 2014 protests.
"People are increasingly losing confidence in the neutrality of Hong Kong's justice system," HRW's China director Sophie Richardson said in a statement.
"Hong Kong authorities should quash the convictions of peaceful protesters that have raised serious concerns about the long-term prospect for human rights and fundamental freedoms in Hong Kong."
If the three are given jail terms over three months, by law they will be barred from running for office for five years, effectively preventing them competing in by-elections to replace Law and five other lawmakers disqualified since December
Antony Dapiran, a lawyer and author of "City of Protest: A recent history of dissent in Hong Kong," said the case "demonstrates a continued politicization of Hong Kong's court system."
"(It raises) concerns that the city's rule of law is being undermined in favor of a system of 'rule by law,' with Beijing using Hong Kong's legal system to serve its political agenda," he said.
In an editorial Tuesday,
the New York Times warned jailing Law, Wong and Chow would "be a watershed in Hong Kong's modern history and should set off further alarms about China's intentions for the territory."
"The promise of 'one country, two systems' is giving way to the reality of one country, one system," the editorial said.