Thousands of Hong Kong residents are locked down for the first time, to contain a worsening outbreak in the city, authorities said.
Hong Kong has been grappling to contain a fresh wave of the coronavirus since November.
Over 4,300 cases have been recorded in the last two months, making up nearly 40 per cent of the city's total.
Coronavirus cases in Hong Kong's Yau Tsim Mong District, a working-class neighbourhood with old buildings and subdivided flats, represent about half of the infections in the past week.
Sewage testing in the area picked up more concentrated traces of the virus, prompting concerns that poorly built plumbing systems and a lack of ventilation in subdivided units may present a possible path for the virus to spread.
Authorities said in a statement that an area comprising 16 buildings in Yau Tsim Mong will be locked down until all residents have been tested.
Residents will not be allowed to leave their homes until they have received their test results to prevent cross-infection.
"Persons subject to compulsory testing are required to stay in their premises until all such persons identified in the area have undergone testing and the test results are mostly ascertained," the Government said in a statement.
Hong Kong has previously avoided lockdowns in the city during the pandemic, with leader Carrie Lam stating in July last year that authorities will avoid taking such "extreme measures" unless it had no other choice.
The restrictions, which were announced at 4:00am local time, are expected to end within 48 hours, the Government said.
The central Chinese city of Wuhan has marked one year since it announced a severe lockdown to contain the coronavirus that was spreading through the city and overwhelming hospitals.
Citizens awoke to news of the lockdown early in the morning of January 23, 2020, just days after a well-known epidemiologist announced the virus was transmissible between humans.
The lockdown was eventually expanded to most of the rest of Hubei province, affecting 56 million people, but the virus had already spread well beyond China's borders.
Wuhan has been largely free from the virus since the end of the lockdown, while smaller outbreaks have set off renewed containment measures in many other Chinese cities.
China reported 107 new COVID-19 cases on the mainland on January 22, up from 103 cases a day earlier, the national health authority said.
The National Health Commission said in a statement that 90 of the new cases were local infections.
The number of new asymptomatic cases, which China does not classify as confirmed cases, fell to 99 from 119 cases a day earlier.
The total number of confirmed COVID-19 cases in mainland China reached 88,911, while the death toll remained unchanged at 4,635.Loading