Thai media reported that roads across the country were packed for the Labour Day weekend, with people travelling after the government announced the first set of eased measures on Thursday.
"Over the past few days, there has been a lot of travelling taking place," army supreme commander General Pornpipat Benyasri said in a press briefing, warning that the public may have misunderstood the situation.
"There are no measures from the government that say you should let your guard down," he added.
Pornpipat said travellers will face health checks such as fever scans at checkpoints on their return trips.
"Please stay home, maybe do some allowed activities, and continue working from home," Pornpipat said. "We have to maintain the low number of infections."
Thailand has been under a state of emergency since March 26. It is set to last until at least May 31. It gives the government sweeping powers to introduce regulations.
A daily curfew from 10 pm to 4 am (1500 to 2100 GMT) has been imposed.
The country has since seen a declining rate of new coronavirus cases.
The government's coronavirus task force reported fewer than 10 new cases a day this week, with three new cases reported on Sunday. It reported more than 100 new cases on most days in March.
It has reported a total of 2,969 coronavirus cases, with 54 deaths.
Businesses allowed to reopen on Sunday have been instructed to incorporate new virus prevention measures.
Restaurants are required to take customers' temperatures and make space between seats. Many restaurants have allowed only one customer per table.
Markets are also required to conduct fever scans.
Barbershops - for both people and pets - are reservation-only, and customers are not allowed to wait inside.
Sports clubs have opened areas for non-contact sports such as running, badminton, tennis, and golf. Football and basketball are still prohibited.
A ban on alcohol sales was also lifted on Sunday, but only for consumption at home, as drinking in shops and restaurants is still prohibited.
Thai media reported crowds of people buying alcohol at shops and supermarkets.
Other places such as shopping malls, bars, libraries, museums, zoos, fitness centres, public swimming pools, day care centres, massage parlours, and playgrounds remain closed.
Officials have said that businesses that are allowed to reopen could be ordered to close again if there are signs of another outbreak. - dpa
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