Samut Sakhon has been put under a lockdown with a curfew imposed from 10pm-5am taking effect last night until Jan 3 in the wake of a new surge of more than 500 Covid-19 cases.
People's movements out of the province are also banned.
The announcement was broadcast on the National Broadcasting Services of Thailand at 9pm.
Opas Karnkawinpong, director-general of the Department of Disease Control, said the 13 local infections initially reported earlier on Saturday later surged to 548 following the testing of 1,192 people in the province. Most are migrant workers.
"So, the rate of infection is 43% from tested people and 90% of them are asymptomatic," said Dr Opas.
"Most of the infected cases are migrant workers," said Dr Opas, admitting the likelihood of more infection cases to be found.
He said there might be more infections especially in the community around the seafood market.
Despite that, Dr Opas said the authority is confident the situation is manageable because the source of the transmission is located and contained. The effect will be minimal as the infections occurred in young and strong migrant workers.
He also asked vendors and buyers who came to visit the seafood market since Dec 1 to go into self-quarantine and monitor their own health for 14 days as they might be at risk from the infection.
Samut Sakhon govenor Verasak Vichitsangsri, in his capacity as chairman of the provincial communicable disease committee, said the new increase in infections was beyond expectations.
The Covid-19 alert level in the province will be elevated starting from Saturday until Jan 3, with the Talad Klang Kung or Central Shrimp Market and Sri Muang dormitory designated as off-limits areas for disease control.
As for other locations in the province, they will be closed temporarily, including all kinds of gambling venues, sport stadiums, boxing stadiums, schools and educational institutes, tutorial schools, shopping malls, cinemas, and spas, among several other venues.
Eateries that sell alcohol are allowed to sell only takeaways while convenience stores are closed between 10pm and 5am.
Earlier in the morning, 12 local infections in Samut Sakhon were among 34 new coronavirus cases reported, raising the nationwide total to 4,331, with no new deaths.
Eight of the 12 cases -- the biggest one-day rise in seven months -- were linked to a 67-year-old shrimp wholesaler who tested positive last week.
The sources of four more local infections in the province are still being investigated.
Three of the eight cases linked to the shrimp wholesaler were asymptomatic close relatives -- her bedridden mother, 96; her sister, 73; and her sister-in-law, 57, according to the Centre for Covid-19 Situation Administration (CCSA).
The woman owns a stall at Talad Klang Kung in Samut Sakhon and fell ill on Dec 13.
The five others in contact with the woman were three workers from Myanmar, aged 40, 32 and 43, and two Thais -- a 24-year-old worker who reported symptoms, and a 41-year-old clerk.
The seafood industry in Samut Sakhon employs thousands of migrant workers from Myanmar.
Public Health Minister Anutin Charnvirakul has previously said the infection could have originated in the local migrant community, given that Myanmar has seen a surge in Covid cases since September.
As of Friday, the neighbouring country had reported 114,000 infections and 2,400 deaths.
Authorities are now investigating how the remaining four local cases reported on Saturday contracted the disease. They were a 42-year-old vendor, two company employees aged 23 and 20, and a 49-year-old cashier.
Meanwhile, Covid-19 cases were also confirmed on Saturday in another 22 quarantined arrivals.