Muang district of Rayong was on Saturday declared a red zone, where the tightest Covid-19 control measures are in place, after 27 new infections were confirmed in a single day, bringing the total number of cases there to 36, provincial governor Chanchana Iamsaeng said.
Eighteen new patients were confirmed at state-run hospitals and another nine at two private-run hospitals, said the governor.
Rayong had been free of Covid-19 for more than 200 days since the first wave of outbreak early this year.
In the new round of the Covid-19 outbreak, the province's first infection was confirmed on Thursday, followed by another eight on Friday and 27 on Saturday.
Under the circumstances, Muang district, where the infections are concentrated, has implemented maximisal Covid-19 control measures. Effective from Monday, all public places which are crowded are ordered to be shut, except for supermarkets.
The new cluster of infections in Rayong has raised the Public Health Ministry's concern of a possible major spread of the virus as an infected person in the province had gambled at a local underground casino prior to testing positive for Covid-19.
The disease outbreak now spans 33 provinces, beginning at the epidemic hotbed in the central shrimp market in Samut Sakhon last week.
Dr Opas Karnkawinpong, director-general of the Department of Disease Control (DDC), on Saturday told the media the gambler was a 55-year-old woman.
She infected a 51-year-old woman who shared a house with her.
The 51-year-old had also been driving a car to meet her sister and nephew.
These individuals caught the virus and passed it to nine others.
Meanwhile, four people suspected of contracting Covid-19 are waiting for the test results.
"We are worried about the cluster because the patient didn't tell the truth," said Dr Opas.
The lack of honesty made it difficult to track the disease. There was no telling how many people were in the casino with the woman at the time, he said.
Thailand on Saturday updated its tally, reporting 6,020 infections with 60 deaths.
Meanwhile, 234 infections in 33 provinces are traced to the central shrimp market in Samut Sakhon.
Dr Sophon Iamsirithavorn, the DDC's acting deputy chief, said it is likely the number of infections may reach 10,000 due to large-scale tests.
Previously, tests were performed only in cases where individuals developed a fever, which could be an early indication of Covid-19. However, 90% of infected people are asymptomatic.
Prime Minister Prayut Chan-o-cha on Saturday exercised his power under the emergency decree to prohibit public gatherings nationwide and authorise governors to decide if a Covid-19 lockdown must be imposed in their provinces, said Dr Taweesilp Visanuyothin, spokesman for the Centre for Covid-19 Situation Administration.
He told a press briefing on Saturday the ban reduces the risk of the virus spreading.
"Basically, provincial governors will decide on their provinces' disease control measures," he said.
"At the same time, the ban on public gatherings is enforced nationwide."
In border district of Mae Sot in Tak, three new Covid-19 cases were confirmed on Saturday, said a source.
One patient was the owner of a bicycle shop and the other two were his employees.
In Krabi, two more cases were detected in a cluster of infections linked with a big bike group that travelled to the Lan Ta island, said a source. In total 17 people in this cluster have been confirmed to have Covid-19.
In Nakhon Pathom, the provincial health office recorded 13 new cases on Saturday, bringing the cumulative number to 30.
Phetchaburi also recorded nine new cases, raising the infection number there to 17 on Saturday. Nakhon Ratchasima recorded three new cases on Saturday, putting the total at six.