Joshua Wong was jailed Wednesday for three months by Hong Kong's high court for an offense stemming from the 2014 Umbrella Movement protests which shut down parts of the city, including the main financial district, for several months.
He was already on bail pending an appeal over a six-month sentence for another protest-related offence.
High Court Judge Andrew Chan said in the ruling that although Wong only stayed in the camp briefly, his involvement in obstruction was deep and extensive.
Raphael Wong Ho-ming, vice-chairman of the League of Social Democrats (left) and Joshua Wong, Demosisto party secretary general speak outside the High Court in Hong Kong on January 17, 2018.
Meanwhile, both activists were taken into custody by security guards. Raphael Wong shouted as he was led away.
Freedom of speech and demonstration is protected by the city's mini-constitution, the Basic Law.
The 21-year-old activist, along with 15 others, had admitted to disobeying the judicial order which asked them to lift the blockade of streets which had lasted 79 days. They displayed a banner calling the activists "mobsters" and saying they must "pay the price" in jail.
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Hong Kong's most famous pro-democracy activist is heading back to prison.
They failed to win concessions and since then leading activists have been charged over their involvement.
Wong's party Demosisto wants self-determination for the city.
Hong Kong was handed over from British rule to China in 1997 with Beijing agreeing to govern the territory under the principle of "one country, two systems".
Under the deal, citizens have the right to freedom of speech and a partially directly elected parliament, as well as an independent judiciary.
It's the latest legal setback for Wong, who has been involved in multiple court cases in the aftermath of the protests including one that also resulted in a prison sentence, which he's appealing. He started serving that sentence in August but was granted bail pending an appeal verdict.