The government order, which also applies to the neighbouring district of Takhmao, was delivered at midnight and came after vague reports of an impending lockdown sparked a run on local markets and stores.
The restrictions apply for two weeks.
Under the measure, businesses deemed non-essential must shut and residents were banned from leaving their homes, with several exceptions.
These included travelling to work for individuals who must leave home for their jobs, trips to buy food three times per week, exercising locally, health care, and urgent trips approved by authorities.
Businesses permitted to remain open including food producers and grocery suppliers, certain factories, hotels and guesthouses, and restaurants providing food for delivery.
Following the rush for supplies by residents on Wednesday evening, Prime Minister Hun Sen released a statement calling for calm.
During the first year of the pandemic, Cambodia recorded less than 500 cases and no deaths. (dpa/NAN)