They are accused of organising or participating in the "Hong Kong Parliament", a group authorities in the Asian financial hub say aimed to subvert state power, under the law Beijing imposed in 2020 following months of pro-democracy protests in 2019.
The activists are accused of having launched a referendum or run as candidates in the unofficial "Hong Kong Parliament" group, which authorities say aims at achieving self-determination and drafting a "Hong Kong constitution".
Police, who said the organisation sought to overthrow the governments of China and Hong Kong by unlawful means, said they are still investigating and further arrests may follow.
Among those named are businessman Elmer Yuen, commentator Victor Ho, and activists Johnny Fok and Tony Choi. Four of them are subject to previous arrest warrants, each carrying a bounty of $HKS1 million ($A192,738).
Among the remaining 15, for each of whom police are offering a bounty of $HK200,000, are those said to have organised or run in the election and sworn in as its councillors.
None of the accused could be reached for comment.
The former British colony returned to Chinese rule in 1997 with the guarantee of a high degree of autonomy, including freedom of speech, under a "one country, two systems" formula.
Critics of the national security law say authorities are using it to stifle dissent.
Chinese and Hong Kong officials have repeatedly said the law was vital to restore stability after the city was rocked for months by sometimes violent anti-government and anti-China protests in 2019.
