About 5,000 troops armed with batons and shields were dispatched to a mountain camp in Huay Nam Khao, north of Bangkok in a pre-dawn operation to clear the 4,400 Hmong.
The Thai army began the forced repatriation of thousands of ethnic Hmong asylum-seekers to Laos on Monday, defying international concerns that some would be persecuted back home.
About 5,000 troops armed with batons and shields were dispatched to a mountain camp in Huay Nam Khao, 300km (186 miles) north of Bangkok in a pre-dawn operation to clear the 4,400 Hmong, who say they face oppression by Laos' communist government if sent back.
Known as America's "forgotten allies", the Hmong sided with the United States during the Vietnam War and many fled Laos in 1975 when the communist Pathet Lao took power. Tens of thousands have since been resettled in the US.
The United States and United Nations have expressed concern about the forced repatriation and the fate awaiting the Hmong once they return to Laos.
The United States on Sunday called on Thailand to halt the operation. "We deeply regret this serious violation of the international humanitarian principles that Thailand has long been known for championing," the US State Department said.
In a statement issued on Sunday, state department spokesman Ian Kelly also urged Laos "to treat humanely any Hmong who are involuntarily returned, to provide access for international monitors, and facilitate resettlement opportunities for any eligible returnee".
"We will remain engaged in this important humanitarian issue," the statement added.
Thailand regards the Hmong in Huay Nam Khao as economic migrants with no claim to refugee status.
Thai prime minister Abhisit Vejjajiva has insisted the deportation would be done humanely. He said the Lao government had assured him the Hmong would be well looked after and the U.N refugee agency UNHCR would be allowed to visit them.
Colonel Thana Charuvat, who is in charge of the repatriation, said troops would try to finish the operation in a day "to prevent any attempted resistance".
"We will try to persuade them. If there is resistance, we will use soft means with self-defence, but we will warn them first," he told Reuters.
New York-based Human Rights Watch said it was worried the repatriation could turn violent.
"There is always resistance, even in small-scale operations. If they resist a major operation like this, the army may react with full force," said Sunai Pasuk, the group's Thailand representative.
Sunai said mobile phone signals had been jammed in the camp to prevent leaders from calling out for help. Reporters were denied access and were kept 12km (7.5 miles) away.
The Hmong were being taken to an immigration centre in Nong Khai bordering Laos before being handed over to Lao authorities, Thana said.
More than 400 were seen boarding some of the 100 trucks sent to the camp in northern Phetchabun province.
Rights groups and UNHCR say some of the Hmong could qualify for refugee status and should not be sent back. Thailand has denied repeated UNHCR requests to visit the camp and determine their status.
The UNHCR said last week Thailand had "the responsibility and international obligation" to ensure those in need of protection back home were repatriated "only on a voluntary basis."
Reporters have been denied access to the camps for the past two years, and in May, Doctors Without Borders (MSF), the main group providing aid to the Hmong, withdrew after criticising how the Hmong were being treated.
Thana said the media had been kept away to prevent the Hmong from carrying out publicity stunts to "draw public attention to their status with self-inflicted physical abuse".
Thailand fears that by facilitating their resettlement in a third country, it could encourages more illegal migrants.
Lionel Rosenblatt, president emeritus of Refugees International, said the deportation could hurt Thailand's reputation.
The country has been a key transit point for as many as 1.5 million refugees from conflicts in Myanmar and Indochina.
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