Hong Kong's highest court has overturned prison sentences for three young pro-democracy activists convicted for their roles in kicking off 2014's "Umbrella Movement" protests in the semi–autonomous Chinese city.

Key points: The trio were originally found guilty of unlawful assembly

The trio were originally found guilty of unlawful assembly Joshua Wong says he will continue "to fight for democracy"

Joshua Wong says he will continue "to fight for democracy" Court says future offenders will face stricter guidelines

A panel of five judges sided with Joshua Wong, Nathan Law and Alex Chow in their appeal against prison terms for unlawful assembly.

The Court of Final Appeal's ruling was a stunning victory for the city's youthful opposition movement after recent setbacks, including one candidate's disqualification from an upcoming election.

"We'll still keep on to fight for democracy. No matter we will be put into prison again or not, now Hong Kong's 'One Country, Two Systems' is still under threat," Mr Wong said.

The case sparked controversy because a magistrate initially gave the trio lenient sentences, but Hong Kong's Department of Justice requested a review that resulted in prison time ranging from six to eight months, raising worries about judicial independence and rule of law.

Mr Wong, 21, Mr Law, 24, and Mr Chow, 27, had already served about two months of their sentences when they were released on bail in November.

The three were convicted of "unlawful assembly" 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 Mr Wong, then still a teen, in the global spotlight.

In a summary of the judgment issued to the media, the five judges unanimously said it had "quashed the sentences of imprisonment" by the lower Court of Appeal.

They stressed, however, that Hong Kong was a law-abiding society and that "future offenders involved in large-scale unlawful assemblies involving violence" will be subject to stricter guidelines.

Hong Kong, a former British colony, returned to Chinese rule in 1997 with a guarantee of wide-ranging freedoms, including freedom of speech, but critics accuse Beijing of creeping interference in the city's affairs and the government of toeing the Beijing line.

AP/Reuters