The lockdown of Sau Ming House, which started at 5 pm, came after the restriction testing orders on two other blocks in the estate ended with the discovery of more than 500 positive cases.
And around the same time, a public housing building in Sha Tin was also locked down.
People in Foo Wo House on Wo Che Estate will be not be able to leave until their test results are out.
Two blocks in Hang Chun Court in Cheung Sha Wan have also been sealed off.
Officials said a number of residents of Chun Lai House and Chun Yin House had recently tested positive for a mutant strain.
The operations are expected to end on Sunday afternoon.