Jimmy Lai's actions seriously endangered national security and Hong Kong's prosperity, Beijing's national security office in Hong Kong said on Monday, as it voiced support for the ruling that found him guilty of three charges.

The Office for Safeguarding National Security said the trial showed the former media tycoon was the "main planner and participant" in a series of anti-China incidents and the "driving force" of the anti-extradition bill movement.

The office also hit out at Lai for asking for foreign sanctions, colluding with organisations advocating "Hong Kong independence" and external forces and abusing mass media to incite hatred and resistance.

"His actions severely violated the bottom line of the 'One Country, Two Systems' principle, seriously endangered national security, and seriously damaged Hong Kong's prosperity, stability as well as the well-being of its people," it said.

Such serious crimes warrant severe punishment, the office added.