At a Sunday press conference, Ms Lam said there had been more than 100 new infections, and announced new restrictions to contain the spread.
She described the situation as "really critical" and said there was "no sign" it was coming under control.
Non-essential civil servants must work from home and testing will increase.
The chief executive promised that authorities would carry out 10,000 tests a day, and also made wearing face masks compulsory in indoor public spaces. Face coverings were already mandatory on public transport.