The city's security chief issued the order under Hong Kong's homegrown national security law, known as Article 23, which was passed last year.

The ban directly prevents the two activist-led organisations -- named Hong Kong Parliament and the Hong Kong Democratic Independence Union -- from operating locally.

It is unclear what impact Tuesday's ban will have, as the groups are largely active on social media, with some members reportedly based abroad.

A statement said the prohibition was necessary for safeguarding national security.

Any activists affiliated with them, or who give them money, face a maximum fine of HK$1 million ($128,000) and imprisonment for 14 years, according to the ban published in the city's law gazette.

Some overseas members are already on Hong Kong police most-wanted lists or have reward notices attached to them.

A 19-year-old woman who took part in two videos to promote the Hong Kong Parliament pleaded guilty to sedition and last month received a one-year prison term.

Beijing imposed a sweeping national security law on Hong Kong in 2020 after the city saw huge and sometimes violent pro-democracy protests the year before.

The former British colony passed an additional, homegrown security law last year, which strengthened penalties for sedition and created five new categories of crimes.

As of the start of November, Hong Kong has arrested 348 people for various national security crimes and convicted 172.