The outbreak began at a shrimp market in Samut Sakhon, which is Bangkok’s southwest province and center of the seafood industry that houses thousands of migrant workers.
“Today is just a first stage,” Kyatifom Wongrajit, the ministry’s permanent secretary, told a news conference. “More results will show more infections.”
He said up to 40,000 people will be tested in Samut Sakhon and neighboring counties, with more than 10,000 tests by Wednesday.
Migrant workers, most of them from Myanmar, lined up for the test on Sunday, along with some Thais. Health officials said most of the cases identified so far were asymptomatic.
Barbed wire surrounded the market on Sunday as authorities in Bangkok ordered the closure of all schools in three districts of the capital that bordered Samut Sakhon, 45 kilometers (30 miles) by road, to close until January 4.
The province is set to remain under lockdown and night curfews until January 3, and Katifom said the ministry expects to bring the situation under control within two to four weeks.
“We have to break the cycle of the epidemic.”
Prime Minister Prayut Chan-ocha posted on his Facebook account: “We will have to quickly cut the cycle of the epidemic. We already have experience dealing with it.”
The authorities in Bangkok also called on people to step up preventive measures by avoiding gatherings, while entertainment venues and restaurants should observe social distancing.
Organizers of New Year’s celebrations have been required to obtain official permission to proceed, while companies have been urged to urge employees to work from home if possible.
Meanwhile, neighboring Cambodia has tightened requirements for people entering the country from Thailand.
Somsak Panetiyasai, president of the Thai Shrimp Association, said the outbreak was bad news for shrimp exports, of which up to 30% come from Samut Sakhon. Thailand is one of the top 10 exporters of shrimp in the world.
The increase in coronavirus cases comes as Thailand tries to revive a tourism industry shattered by the epidemic. Thailand on Thursday eased restrictions to allow more foreign tourists to return.
Co-reporting by CNN’s Kucha Ularen and Nectar Jan.