Since taking power in a 2014 coup, Thailand's ruling junta has attained varying degrees of success in campaigns to regulate the foreign workforce, spurred partly by media reports that unregulated workers faced exploitation by employers.
About 60,000 workers left between June 23 and 28, and the number has risen since, an Immigration Bureau official said.
"They were of all nationalities, but the biggest group was from Myanmar," Deputy Commissioner Pornchai Kuntee told Reuters. "They are probably very scared."
Following news of the exodus, Thailand on Friday promised a 120-day delay in enforcing parts of the decree, including fines that can range up to 800,000 baht ($23,557) for employers who hire unregistered foreign workers without permits.
Geta Devi, 28, a Myanmar worker based in the Thai capital of Bangkok, said some of her friends panicked over the decree.
"They went back to Myanmar," she added.
Since last week, up to 500 Cambodian migrant workers have returned home, said Chin Piseth, deputy chief of the Thai-Cambodia border relations office of the Cambodian army.
"According to reports I received, between 400 and 500 were deported," he told Reuters.
The mass movement leaves undocumented workers vulnerable, said Andy Hall, a British specialist in migrant workers' rights who has monitored such migration in Thailand for more than a decade.
"It's clear to me tens of thousands of migrants only move like this after instigation," Hall, who has worked extensively with Myanmar workers, told Reuters.
Despite the threat of punishment, "corrupt officials" would try to seek bribes, he said, adding, "Mass profit is to be made in a short time from the panic and commotion."Police trying to extort money from employers or migrant workers face punishment, Thai police chief Chaktip Chaijinda said on Friday, in a bid to discourage such exploitation.
Last month, the United States kept Thailand on a trafficking watch list, saying it did not meet the minimum standards to end human trafficking.
Thailand defended its efforts to stop trafficking and urged U.S. officials to visit and gauge its campaign.
More than 3 million migrants work in Thailand, the International Organization for Migration says, but rights groups put the figure higher.
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