The pro-democracy candidates had hoped to obtain a majority in the upcoming elections.
At least 12 Hong Kong opposition candidates were disqualified from a legislative election set to take place in September.
Authorities said Thursday they had not upheld the city's mini-constitution and pledge allegiance to Hong Kong and Beijing.
Among the candidates was pro-democracy activist Joshua Wong.
"The excuse they use is that I describe #nationalsecuritylaw as a draconian law, which shows that I do not support this sweeping law," Wong tweeted.
Prominent pro-democracy activist Tiffany Yuen from the political organisation Demosisto, incumbent lawmaker Dennis Kwok and three others from the pro-democracy Civic Party were also disqualified.
Other nominations are being reviewed, and the government expressed support for the disqualifications.
The disqualifications are a setback for pro-democracy activists, who aimed to win a majority of seats in the legislative council. Candidates like Wong topped polls in an unofficial primary recently.
Earlier Thursday, pro-democracy activist Lee Cheuk-yan criticised the new national security law imposed by China following protests in Hong Kong that went on for months.
“Hong Kong politics keeps changing,” said Lee, according to AP. “Now they are using the national security law against the young people … these young people are being charged just for the things they said.”
Authorities had arrested four young people Wednesday for announcing plans to set up an organisation that supported independence.