About a third of Vietnam's 100 million people must now stay home as multiple southern provinces went under Covid-19 lockdown on Monday.
The restrictions come a day after nearly 6,000 new infections were reported over 24 hours, a record for Vietnam.
Residents in the capital Hanoi were also asked -- but not ordered -- to remain at home, and all non-essential shops and services were forced to close. Gatherings of more than five people in public places are banned.
"The order from Hanoi authorities came so suddenly," said resident Nguyen Thanh Van.
"But I fully agree with it. We'd rather they come down hard to avoid a similar scenario to Ho Chi Minh City.
"We won in the previous battles. But this is a hard one."
Business hub Ho Chi Minh City -- where most new infections have been recorded -- and the nearby Binh Duong and Dong Nai provinces have been in lockdown for more than a week.
But cases kept rising. In response, the government ordered 16 more provinces into lockdown starting Monday.
After successfully containing coronavirus outbreaks last year, Vietnam has seen cases skyrocket since late April, with the epicentre moving south from the industrial areas in the north.
In Ho Chi Minh City, authorities have built several makeshift hospitals that can host thousands of patients.
The number of domestic flights to the area has been reduced.
As restrictions tightened, the city's residents complained that getting essential goods was difficult after wholesale and open-air markets closed.
At present, "we give priority to protecting people's health," said Prime Minister Pham Minh Chinh, according to the government's official news site.
The Southeast Asian country has been slow to procure and administer vaccines, with just 4.3 million doses given so far.
Vietnam is also developing its own shots, and hopes to reach herd immunity by early 2022, authorities said.
A total of 55,845 cases and 254 deaths was recorded on Monday.
Backed by its successful containment of the virus, Vietnam was one of the few economies to expand last year.
The economy grew 5.64 percent, in line with government expectations, in the first half of the year.