A top security official on Friday said illegal activities of any kind must be condemned, and called on anyone feeling threatened to come forward after the Hong Kong Journalists Association reported what it called a "systematic and organised attack against journalists" in the city.

The industry body said dozens of journalists had reported being harassed and intimated, with the individuals - and their family members - facing threats on social media, via email or letters between June and August this year.

Michael Cheuk, the under secretary for security, said the people in question should inform the authorities.

"Hong Kong is a place of rule of law. We condemn all sorts of actions that should be outlawed. Any individual should be free from threats, free from fear, and free from harassment... Irrespective of who you are, if you are receiving such threat or harassment, please come out and report," Cheuk said.

"Anyone's freedom, not just about freedom of the press, free from fear, freedom of expressions, whatsoever, those rights should be respected and should not be compromised."

The association also raised concerns about an apparent government data leak, saying certain individuals received threatening messages shortly after being searched by customs officers upon entering Hong Kong.

"I don't think there's any basis of that allegation. At the moment, we haven't received any information that our database of any individual department has been compromised," Cheuk said.