More than 1,000 people protested near the government offices Friday night in response to the decision.
Hong Kong's high-court has ruled in favor of expelling four opposition lawmakers from the city's legislature in a case that critics say calls the territory's independence into question.
The Chinese territory's Court of First Instance ruled the four had not been honest when they altered their oaths, and declared their seats vacant.
The unprecedented intervention was prompted by a string of protests during the swearing in of lawmakers the month before.
The court's decision comes just two weeks after the installation of pro-Beijing chief executive Carrie Lam and the visit of China's president, Xi Jinping, to mark 20 years since the territory reverted from British to Chinese rule.
The atmosphere is decidedly muted in the democracy camp in Hong Kong since the handover anniversary events, where Mr Xi warned democracy advocates against crossing a "red line" and undermining Beijing's authority.
Mabel Au, director of Amnesty International Hong Kong, said in a statement that Friday's decision "confirms the Hong Kong government's agenda to silence and effectively punish any speech critical of the present political system, wherever it may occur, even within the legislature".
"Everyone in their speeches said although it feels hopeless, we need to persist... but they can't seem to say how it's possible to stop these things from happening again", said Wong. China promised Hong Kong autonomy from the mainland, but there are signs of increased involvement by Beijing in Hong Kong affairs.
Veteran activist Leung Kwok-hung raised a yellow umbrella - a symbol of the democracy movement - which the court said did not reflect the "importance and seriousness" of the ceremony.
But the bloc shrunk in November after the disqualification of the two activists. He was accused of adding words to his oath and for reciting the oath in a tone that "expressed a doubt on or disrespect of the status of the [People's Republic of China] as a legitimate sovereign of the Hong Kong Special Administrative Region", according to the court. Now with 24 seats - one held by a centrist lawmaker - and with four more of them facing legal battles - the opposition risks losing its veto power.
The four had originally been allowed to retake their oaths, which were accepted, unlike the other two who were disqualified previous year without a second chance. Both had declined to take their oath and also displayed banners that read "Hong Kong Is Not China".
Law had added a quote from Gandhi, while Yiu added a sentence saying he will "uphold procedural justice in Hong Kong, fight for genuine universal suffrage and serve the city's sustainable development".