Hong Kong police unveiled a dedicated hotline on Thursday for residents to report national security threats, sparking criticism that the measure will deepen swirling distrust in the already polarised city.
China imposed a sweeping new national security law in June after last year's huge and often violent pro-democracy protests in the financial hub.
Despite assurances that the law would only target a "tiny minority", its wording has outlawed a host of peaceful political views and helped to stamp out mass dissent in the financial hub.
The hotline allows residents to send "national security intelligence" via text message, email and the Chinese messaging app WeChat.
Hong Kong residents can also use the line to send pictures, audio and video files, police said in a Thursday Facebook post.
Rights groups have voiced concerns that the hotline will deliver another blow to free speech at a time when Beijing is ramping up control over the city.
"Informants may use this hotline against people who they dislike or are in a different political camp," Human Rights Watch senior researcher Maya Wang told AFP.
Opponents have drawn parallels with the Cultural Revolution in mainland China, a turbulent period in the 1960s and 70s when millions were punished and purged by authorities -- often after denunciation by family, friends and neighbours.
The police post on Facebook had more than 700 comments on Thursday afternoon, with some welcoming and others condemning the initiative.
"Great! Now the cockroaches have nowhere to run," wrote one user, using a pejorative term for democracy supporters.
Beijing's new security law bypassed Hong Kong's legislature and was only revealed on the day it was enacted.
Critics say its broadly worded provisions are a hammer blow to the flickering freedoms that China promised Hong Kong could keep after the end of British colonial rule in 1997.