The sentences handed down by the city’s Court of Appeal came as fears grow that Beijing is tightening its grip on the semi-autonomous city and that rule of law is being compromised.
Wong, Nathan Law and Alex Chow were given terms of six months, eight months and seven months respectively after the court upped their previous non-custodial sentences.
“The court has a responsibility to send out a clear message to society, that in activities such as rallies, marches and protests, when rights are freely exercised, participants must abide by the law,” the judgement said.
Anyone who receives a jail term of more than three months is barred from running for Hong Kong’s partially directly elected parliament for five years.
Wong turns 21 in October which would have made him eligible to run for lawmaker, something he had said he wanted to do.
He told AFP ahead of the judgement that he believed the case was a bid by authorities to deter youth activists.
As he was led away by security, Wong shouted: “Hong Kong people, don’t give up!”.
Law, 24, was voted in as a legislator by the public last year, winning 50,000 votes in what was seen as a victory for the democracy movement.
But he was disqualified last month along with three other pro-democracy lawmakers for inserting protests into their oaths of office.
That decision came after an unprecedented intervention from Beijing demanding oaths are “solemn and sincere”.
‘Vindictive attack’
Hong Kong enjoys much greater freedoms than on the mainland, with its liberties enshrined in the handover agreement made when the city was given back to China by Britain in 1997.
But there are growing concerns Beijing is trampling the deal.
The Umbrella trio were found guilty last year on unlawful assembly charges for storming a fenced-off government forecourt known as “Civic Square” as part of a protest calling for fully free leadership elections in September 2014.
Their arrests sparked wider rallies which exploded two days later when police fired tear gas on the crowds, triggering mass demonstrations that brought parts of Hong Kong to a standstill for more than two months in an unprecedented challenge to Beijing.
Wong and Law received community sentences and Chow a three-week suspended sentence at magistrates’ court last August over the Civic Square protest.
But Hong Kong’s justice department then sought to increase those terms, with prosecutors arguing they should receive harsher punishment.
In emotional scenes before the judgement, tearful supporters hugged the three and chanted: “Shame on political persecution!”
Around 20 pro-Beijing supporters shouted back through loudspeakers, chanting “Pay the price in jail!”
After the court’s decision supporters outside were in tears.
The justice department said there was “absolutely no basis to imply any political motive” on its part in relation to the case in a statement Thursday morning.
However, Amnesty International slammed authorities’ pursuit of jail terms for the trio as a “vindictive attack on freedom of expression and peaceful assembly”.
It described the public order regulations on which the case was based as “vague” and said they had repeatedly been criticised by the United Nations Human Rights Committee for failing to fully meet international human rights law and standards on the right of peaceful assembly.