The ambush-style lock-down covers the building on 271-285 Des Voeux Road West, all residents must get tested for the virus before 2am and stay home until the operation is complete.

A government spokesman said authorities are concerned that the infections from the gym cluster may have spread to nearby buildings, especially in old buildings with subdivided units within.

Officials say they expect the operation to be completed by 7am on Thursday to allow residents to go to work.