The protesters, most of whom were wearing the white, smiling Guy Fawkes masks made popular by anti-establishment hackers, the film "V for Vendetta", chanted slogans against the Hong Kong police and in favour of democratic reform in the Chinese-ruled territory.
After the flash mob ended, some protesters constructed makeshift barricades and vandalised a nearby restaurant.
Hong Kong's economy has been weighed down by months of protests, many of which have turned violent with frequent episodes of radical demonstrators setting fires and vandalising the public transport system.
The protesters accuse Beijing of interfering more and more with Hong Kong, which returned from British to Chinese rule under a "one country, two systems" formula intended to guarantee freedoms that mainland citizens do not have.
China denies meddling and has accused foreign governments, including the United States and Britain, of stirring trouble.
Guy Fawkes Day, also called Bonfire Night, is celebrated with fireworks and bonfires every Nov.
5 in Britain.
Effigies of "guys" are burnt, marking the night in 1605 when Fawkes was arrested for a "gunpowder plot" to blow up parliament