Chinese and Indians are among the foreigners taking jobs reserved for Thais, according to the Department of Employment.
The department had launched random inspections of workplaces across the country after receiving complaints about jobs being taken illegally by foreigners.
From Oct 1 to March 18, a total of 25,628 workplaces were inspected, with 820 facing legal action for allowing violations of the labour law.
The workplaces inspected employed a total of 306,577 foreign nationals. The figure included 232,106 from Myanmar, 42,698 from Cambodia, 18,001 from Laos and 236 from Vietnam.
A total of 1,689 workers faced legal action, with 721 found to be doing jobs that are off-limits to foreigners.
Of the violators, 316 were from Myanmar, 175 from Cambodia, 106 from Laos, 65 from India, 42 from Vietnam and five from China.
Among the restricted jobs being done by non-Thais were street hawkers, barbers, public transport drivers, and traditional masseuses and masseurs.
Some of the foreign workers were hired in non-restricted jobs but failed to comply with rules, the department said, citing such positions as store salespersons and construction workers.
Meanwhile, a Move Forward Party MP for Phuket wants officials to inspect shady businesses in the island province after receiving complaints about them.
Some illegal businesses were found to be operated by foreign nationals on long-stay visitor visas, the MP said, notably real estate firms, that are suspected of using Thai nominees.
