The Human Rights and Development Foundation says Thai authorities are targeting for arrest migrant workers who failed to take part in a nationality verification program.
The HRDF says the government this month set up a special center to manage a crackdown on about 300,000 migrant workers who missed a February deadline to begin the verification process.
The rights group says hundreds of migrants have already been arrested and more arrests are expected.
Somchai Homlaor is a human rights lawyer and secretary general of the HRDF. He says the crackdown and high demand for cheap migrant labor will only encourage bribes and other criminal activities that the registration program was meant to prevent. He says the deadline to apply for nationality verification should be extended.
"The Thai government should open for the registering of these illegal migrant workers and allow them to become workers who work in Thailand legally, that they will not [be] subject to the exploitation and abuse of the power of the authority," he said.
The nationality verification program is part of the government's effort to give migrant workers legal protections and better access to public services.
There are more than two million migrant workers in Thailand, many of them illegally or without proper documentation. More than 80 percent are from Burma.
Their access to education and health care is limited and they are often taken advantage of by crooked employers, but have few legal remedies.
Only legal migrants were allowed to participate in the verification process. About 800,000 from Burma applied while an estimated one million unregistered were excluded and are subject to arrest and deportation.
Under the program, migrant workers are required to verify their nationality with their home government before they will be issued a work permit in Thailand.
But Somchai says Burma's military government does not recognize some ethnic groups, such as the Rohingya, a Muslim minority, as citizens of Burma.
"There are many, many workers from Burma that will not be able to pass their national verification process and they will not get the passport from Burmese authority and will not get the work permit from the Thai authority. This is the big question that what Thailand will do for this group of [what] becomes the stateless persons. We don't have a clear answer from the Thai government even though we raised this issue some time ago," he said.
Somchai says many migrants from Burma also refused to register for fear of persecution from Burmese authorities.
Others, he says, could not afford fees for brokers who help migrant workers through the application process or were not aware of the verification requirement.
Thailand depends on migrants as a cheap source of manual labor. Many work on construction sites and fishing boats, or as household servants.