After months of avoiding the surge in cases seen by its neighbours, Thailand has been hit by its worst Covid-19 outbreak yet.
Tens of thousands of people are being tested after hundreds of cases linked to its biggest seafood market.
It has locked down Samut Sakhon, the coastal province near the capital Bangkok, home to the market which employs mostly migrant workers from neighbouring Myanmar.
Workers have been ordered to stay home.
Thailand was the first country outside China to register cases of Covid-19. But it has recorded a relatively low virus caseload since, with just over 4,000 cases and 60 deaths before this latest outbreak.
How did this surge begin?
It started with a 67-year-old Thai female prawn seller testing positive on Thursday.
Her case at Mahachai market - the heart of Thailand's multi-billion-dollar seafood industry - set the mass testing in motion.
Four cases were reported the next day and by Sunday that number had jumped to 689 - most of them Myanmar migrants, the public health ministry said.
It added that the woman had no history of leaving the country and authorities were searching for the source of the infection.
What has the response been?
Along with stepping up testing, Myanmar workers who live around the market have been ordered not to leave their residences, according to AFP news agency.
"We are locking and banning them from moving," the health ministry's permanent secretary Kietphgum Wongit said, adding that the workers were being provided with food and water.
Lines of migrant workers - mostly from Myanmar - were seen waiting for their coronavirus swab in the market on Sunday, alongside some Thais.
Health officials said most of the cases identified so far have been asymptomatic, reported Reuters news agency.
A further 360 migrant workers tested positive on Monday - the largest daily infection numbers for Covid-19 ever recorded in Thailand.
Taweesin Visanuyothin, a spokesman for Thailand's Covid-19 taskforce, said authorities would also be conducting "active tracing in several communities of about 10,300 people", AFP said.
Samut Sakhon will stay under lockdown and night-time curfew until 3 January.
Authorities have said up to 40,000 people will be tested there and in nearby provinces.
How will it impact the country?
Thailand had largely appeared to have kept Covid-19 in check, recording a low death toll for a nation of 70 million people that shares porous borders with four countries, including Myanmar.
"We will have to cut the epidemic cycle quickly. We already have experience in handling it," Thai Prime Minister Prayuth Chan-ocha posted on his Facebook account over the weekend.
A Myanmar worker who has helped coordinate testing in Samut Sakhon told AFP that anti-migrant sentiment had been shared on social media - with Thais accusing the workers of importing Covid-19.
In Myanmar the nation's already fragile healthcare system has been overwhelmed by a spike in coronavirus cases after a big second wave hit the country in mid-August - with daily cases rising above 1,000 in recent months.
Authorities in Bangkok have urged people to intensify preventative measures against the coronavirus by avoiding gatherings and observing social distancing. They have also asked public and private sector organisations to cancel their annual new year countdown parties.
The latest outbreak comes as Thailand attempts to revive a tourist industry hit hard by the pandemic. On Thursday it had eased restrictions to permit more foreigners to visit the kingdom.