The new tactic involves authorities declaring lockdown without warning in a bid to ensure people don't flee before Covid-19 testers arrive in their neighbourhoods.
Hong Kong has begun using "ambush lockdowns" to suddenly close off and test everyone inside neighbourhoods where coronavirus cases have spiked as a spate of recent outbreaks lay bare the rampant inequality in the wealthy Chinese finance hub.
Police cordoned off a row of densely packed tenement buildings in the Yau Ma Tei area overnight on Tuesday through to Wednesday morning to conduct mandatory tests.
The new tactic involves authorities giving no warning of an impending lockdown.
City leader Carrie Lam said such "ambush style" lockdowns were needed to ensure people did not flee before testers move in.
"I thank residents in the restricted area for their cooperation," she wrote on her Facebook page Wednesday as the lockdown was lifted.
A similar two-day lockdown in a neighbourhood over the weekend was leaked to the media a day before police moved in.
Tuesday night's operation was small.
Some 330 tests were conducted in 20 buildings, with one coronavirus case found.
But authorities say further ambush lockdowns may be necessary in the days ahead.
Hong Kong was one of the first places to be struck by the coronavirus after it spilled out of central China.
It has recorded just over 10,000 infections with some 170 deaths by imposing effective but economically ruinous social distancing measures for much of the last year.
In recent weeks stubborn clusters have emerged in low-income neighbourhoods notorious for some of the world's most cramped housing.
On paper Hong Kong is one of the richest cities in the world.
But it suffers from pervasive inequality, an acute housing shortage and eye-watering rents that successive administrations have failed to solve.
The average flat in Hong Kong is about 500 square feet (46 square metres) and sells for around HK$7 million ($900,000).
Rents are punishing. Many therefore squeeze themselves into even smaller subdivided flats known as "cage homes", cubicles that can be as tiny as 50 square feet or even less, with shared bathrooms and showers inside ageing walk-up buildings.
It is in these kinds of building where many clusters have been located in recent weeks.