The Court of Appeal in Hong Kong sentenced three student activists on Thursday to jail terms of six to eight months for their roles in the "Occupy Central" movement in 2014, after the government sought to toughen their original sentences.

The trio - Joshua Wong Chi-fung, Nathan Law Kwunchung and Alex Chow Yongkang - led more than 100 protesters who stormed into the forecourt of the east wing of the government headquarters in Admiralty district two days before the protest broke out in September 2014.

In the 64-page judgment passed down on Thursday, Vice-President of the Court of Appeal Wally Yeung Chun-kuen said the crime was "severe" and "needed to be deterred".

The court held that immediate imprisonment was the only appropriate punishment, as the three sabotaged public order and social stability, while claiming that all they did was "exercise their rights of organizing assembly and protests".

Wong and Chow, who were found guilty on one count of taking part in an unlawful assembly, were ordered jailed for six and seven months respectively, while Law was sentenced to eight months for one count of inciting others to participate in an unlawful assembly.

In August last year, the Court of First Instance sentenced Wong and Law to 80 to 120 hours of community service respectively; Chow was sentenced to a three-week jail term, suspended for one year.

The Department of Justice sought to overturn the ruling last week. It argued that the sentences were not sufficient punishment for the violence committed in the assembly and would send a wrong message to the city's youth.

The unlawful assembly took place on Sept 26, 2014, two days before the 79-day "Occupy Central" movement began. More than 100 protesters stormed into the square outside the government headquarters by climbing a three-meter metal fence erected to keep them out.

Starry Lee Wai-king, chairwoman of the city's biggest political party, the Democratic Alliance for the Betterment and Progress of Hong Kong, said the increased sentences have sent a clear message to society. Some people may have a wrong perception that one can do anything to pursue an ideal or a purpose, but the misconception is hazardous for society, Lee said. The sentence would reaffirm the rule of law, she said.

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