Police have arrested three people on suspicion of possessing imitation firearms, weapons and what they described as anti-government material.
Officers said there's no evidence to suggest the suspects knew one another or the cases were linked.
The suspects, aged between 17 and 44, include a secondary school student. Police said they raided their homes after receiving intelligence.
Officers said they found items such as air guns, gas masks, flags that read "Liberate Hong Kong, Revolution of Our Time" and "anti-government newspapers" in the teenager's home.
Chief inspector Chan Ka-ying from the New Territories South regional crime unit said: "The content in the flags is anti-government. We will further investigate the motive of this arrested person: why did he store such items at home and whether [he] has any unlawful purposes and any planned illegal activities in future."
When asked if it's against the law to possess the flags in question, senior superintendent Tony Ho from the Organised Crime and Triad Bureau said: "All cases should be looked at individually and assessed by the merits of the available evidence. It's difficult to answer some questions without any other relating contexts."
In all, the police seized 38 imitation firearms, eight knives, one extendable baton, some gas masks, filters and pro-Hong Kong independence flags.