Hong Kong activist Joshua Wong has been jailed for six months for taking part in the 2014 pro-democracy protests.
He was tried and found guilty of unlawful assembly last year, and sentenced to community service.
But in an unusual move the Hong Kong government appealed the sentence, arguing it was too lenient.
The sentence means he cannot stand for local elections in the next five years. Two other student leaders, Nathan Law and Alex Chow, were also jailed.
Law was sentenced to eight months in prison, while Chow received a seven-month sentence.
The BBC's Juliana Liu in Hong Kong says rights groups have criticised the Hong Kong government, saying the appeals were political decisions intended to deter future protests and to keep young people out of elected politics.
But the Hong Kong government has denied this, saying there was "absolutely no basis" to claims of political motives.
'Lock up our bodies, not our minds'
Immediately after the sentence was announced, Wong - who is 20 - took to his Twitter account with a series of defiant messages.
He ended his series of tweets with a brief "see you soon".
The three activists were convicted for unlawful assembly in an incident which helped to trigger the mass protests in Hong Kong.
They were among a group of student protesters who scaled a fence surrounding Hong Kong's legislative headquarters and occupied the building's courtyard on 26 September 2014.
Their removal by police officers angered the public, and brought tens of thousands of people onto the city's streets in the following days.
The activists initially received community service sentences. Their jail sentences effectively prevent them from standing in upcoming elections, which they had previously expressed interest in contesting.
Anyone jailed for more than three months is disqualified from contesting local elections in Hong Kong for the following five years.
Mr Law was elected to Hong Kong's legislature last year, but was disqualified last month when the city's high court ruled that he had improperly taken his oath.
Amnesty International called the Hong Kong authorities' appeal for jail terms for the three activists a "vindictive attack" on freedom of expression.
"The relentless and vindictive pursuit of student leaders using vague charges smacks of political payback by the authorities," said Mabel Au, director of the rights group's Hong Kong branch.
The justice department said in a statement released prior to Thursday's sentencing that "there is absolutely no basis to imply any political motive" on their part, and that freedom of speech is protected in Hong Kong.
It added that the activists had shown "disorderly and intimidating behaviour", and were convicted "not because they exercised their civil liberties but because their relevant conduct in the protest broke the law".