The “restriction-testing declaration” covered all five blocks of the estate and was expected to finish by 10am on Friday.
Earlier in the day, the University of Hong Kong’s Faculty of Medicine warned of a potentially large Covid-19 cluster developing in Kennedy Town.
“We would strongly advise all students and staff to refrain from visiting the area for lunch or engaging in any mask-off activities for the rest of this week,” the faculty wrote in a social media post.
The government said the Sai Wan Estate lockdown was based on factors including sewage sampling and relevant positive cases.
It also announced that it was delivering rapid test kits to more than a dozen compounds and buildings in Kennedy Town and appealed to residents to test themselves for the coronavirus.
The residential compounds covered include Smithfield Terrace, Smithfield Court, Kwun Lung Lau, The Hudson, Mount Davis 33 and The University of Hong Kong Jockey Club Student Village III.
It said sewage samples collected from the blocks indicated that there may be hidden cases.