On Wednesday, the Court of First Instance sentenced Raphael Wong and Joshua Wong to 4.5 months and three months in prison respectively over the involvement in the clearance of a Mong Kok Occupy site in 2014.
Before the hearing, Wong said he had "no regrets".
"They can lock up our bodies but they can't lock up our minds", he said outside the court.
Wong was charged with failing to comply with a court order to clear a road in the protest.
Leaders of the 79-day civil disobedience movement rejected a August 31, 2014 decree from China's parliament, the National People's Congress (NPC), which required the vetting of candidates for the city's top job by a pro-Beijing committee, as "fake universal suffrage".
The Umbrella Movement was an unprecedented rebuke to Beijing as tens of thousands of protesters brought parts of the city to a standstill demanding fully free leadership elections to replace a system where the chief executive is selected by a pro-Beijing committee.
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Fourteen other defendants including leading activist Lester Shum were given suspended sentences on contempt charges.
The name of the movement arose from the use of umbrellas as a tool for peaceful resistance to the Hong Kong police during the protests.
As the judge said he had no power to grant bail to the duo, they were sent to a correctional facility in Lai Chi Kok Wednesday evening to serve time.
"We want genuine universal suffrage!" he said.
The former British colony returned to Chinese rule in 1997 amid promises that it would keep a high degree of autonomy, but activists accuse Beijing of tightening control in recent years. "However, it may well deter others from participating", he adds, by reinforcing the message that "the pro-democracy camp is not welcome". Joshua Wong's Demosisto colleague Agnes Chow will contest the Hogn Kong Island seat.
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. The city's highest court began hearing an appeal of that prison term on Tuesday.