A panel of judges sided with Joshua Wong, Nathan Law and Alex Chow in their appeal against months-long prison terms for unlawful assembly.
The Court of Final Appeal's ruling was a stunning victory for semi-autonomous Chinese city's youthful opposition movement after recent setbacks, including one candidate's disqualification from an upcoming election.
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The case sparked controversy because a magistrate initially gave the three lenient sentences but the justice secretary requested a review that resulted in prison time ranging from six to eight months, raising worries about judicial independence and rule of law.
Wong, 21, Law, 24, and Chow, 27, had already served about two months of their sentences when they were bailed.
The trio were convicted on unlawful assembly charges for their part in storming a courtyard at government headquarters in September 2014 to protest Beijing's plan to restrict elections, kicking off Hong Kong's most turbulent period in decades and putting Wong, then still a teen, in the global spotlight.
The decision by the five-judge panel promises to reinvigorate the youthful opposition movement that emerged from the aftermath of the 11-week protests after recent setbacks.
Last month, election officials barred 21-year-old Agnes Chow, a member of Wong and Law's political party, Demosisto, from running for an upcoming election, saying their party's political platform advocating self-determination or independence for Hong Kong violated the city's constitution.
Wong may still end up behind bars. He is also appealing a three-month prison sentence for a separate contempt case related to the 2014 protests.
Last week a dozen U.S. lawmakers nominated Wong, Law, and Chow along with Hong Kong's entire pro-democracy movement for the Nobel Peace Prize, in an effort to recognize what they said were peaceful efforts to bring political reform to Hong Kong and uphold its rule of law and human rights.