The workers will be paid - but some have complained of overcrowded and dirty conditions.
Singapore has told 20,000 foreign workers to stay in their dormitories for 14 days as coronavirus cases increase in the city state.
Two dormitories have been isolated: one with 13,000 workers and 63 cases, and one with 6,800 workers and 28 cases.
They are typically home to men from South Asia who work in construction.
The workers will be paid and given three meals a day - but some have complained of overcrowded and dirty conditions.
Although Singapore was praised for its proactive response to the virus, the number of locally-transmitted cases is increasing - and a "lockdown" begins on Tuesday.
What has the government announced?
Two places are being sealed off - the S11 Dormitory at Punggol and the smaller Westlite Toh Guan dormitory.
The government said cases in the dormitories were rising, and that isolation would "keep the workers safe [and] protect the wider community from widespread transmission".
Workers are banned from leaving their blocks, and have been told not to mix with people who don't live in their room or floor.
The number of people in a room varies between dormitories, but in 2015 the BBC visited a new complex which had 12 people per room.
What are conditions like?
The workers will get their salaries, and their employers are entitled to claim S$100 a day (£57, $67) to cover their wages.
They will get three meals a day, plus masks, hand sanitisers, and thermometers. Temperatures must be recorded twice a day and there will be "on site medical support".
But six workers in the Punggol dormitory told the Straits Times their centre already had cockroaches, overflowing toilets, and queues for food.
Foreign worker dormitories are privately-run but must be licensed if they have more than 1,000 occupants. There are around 40 such places in Singapore.
What is the virus situation in Singapore?
Singapore has recorded more than 1,300 cases and six deaths.
Although most cases were initially imported, local transmission is rising and on Sunday, 120 new cases were confirmed - the highest daily rise so far.
On Friday, the government announced a so-called "circuit breaker" policy, similar to the lockdowns seen in other countries.
Schools and non-essential businesses are set to close this week, and people have been encouraged to stay at home.
How many foreign workers are there in Singapore?
Some 5.7m people live in the tiny city-state, including 1.4m foreign workers (as of June 2019).
Of the foreign workers, 284,000 are there under "construction work permits".
Employers can apply for construction permits for workers from 12 places, including India, Bangladesh, Sri Lanka, and Thailand. BBC