Demosisto, a pro-democracy group led by Hong Kong activist Joshua Wong, disbanded hours after the legislation was passed, while prominent group member Nathan Law said on Friday that he had left the global financial hub.
Al Jazeera's Sarah Clarke, reporting from Hong Kong, said Law did not close his whereabouts for security reasons and is "just one among a number of political figures who've fled as a result of the national security law".
Officials in Beijing and Hong Kong say the law, which bypasses Hong Kong's legislature, is necessary to restore order and stability in the city and will only target a handful of "trouble-makers".
Meanwhile, Hong Kong's local government confirmed that a popular protest slogan used over the last year - "Liberate Hong Kong, revolution of our times" - was now illegal.
"The slogan 'Liberate Hong Kong, the revolution of our times' nowadays connotes "Hong Kong independence", or separating the Hong Kong Special Administrative Region (HKSAR) from the People's Republic of China, altering the legal status of the HKSAR, or subverting the state power," the government said in a statement late on Thursday
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