Eight men and four women, aged from 32 to 62, were arrested by police in a drug den in Mong Kok in Kowloon in the early hours of Tuesday.
At 2am, a 39-year-old local man surnamed Yu was arrested in an apartment on 55 Dundas Street for allegedly operating a drug den and selling illicit substances, Oriental Daily reported.
Officers from the crime unit at West Kowloon Regional Police Headquarters took the rest of the suspects, including a pregnant woman and four non-Chinese people, into custody for suspected drug possession and smoking or inhaling drugs.
Some 26 grams of suspected methamphetamine, 13 grams of cocaine, nine midazolam tablets with an estimated street value of HK$23,000 (US$2,941) were found in the flat.
Some tools suspected to be used to smoke the drug, an electronic scale and tools for packaging were seized.
It was understood that some of the people arrested have triad backgrounds.