More than 110,000 Cambodians have fled Thailand to return home in the past week amid fears of a crackdown on migrant workers after last month's military takeover.
Labourers from Cambodia, Laos and Myanmar play a key role in Thai industries such as seafood, agriculture and construction, but they often lack official work permits.
On Wednesday Thailand's military regime, which seized power in a coup on 22 May, threatened to arrest and deport all illegal foreign workers.
"They're returning en masse like a dam collapsing. They've never come en masse like this before in our history," Kor Sam Saroeut, governor of northwestern Banteay Meanchey province where the main Cambodian-Thai border crossing is located, told AFP by telephone.
More than 110,000 Cambodian migrants had returned from Thailand in the last week as of Sunday morning, many of them transported to the border by the Thai military, he said.
"They said they are scared of being arrested or shot if they run when Thai authorities check their houses," he added.
"Most of them went to work in Thailand without a work permit."
Cambodian authorities have arranged nearly 300 cars and military trucks to transport workers from the Aranyaprathet-Poipet border checkpoint to their homes.
Thai military officials were not immediately available for comment on the mass exodus.
Soum Chankea, a coordinator for Cambodian rights group ADHOC who has met many workers at the border, said the number of migrants returning home was growing each day.
"They keep coming, more and more. Thousands more have arrived in Poipet this morning," he told AFP by telephone.
Six Cambodian workers and a Thai driver transporting them to the border province of Sa Kaeo died in an accident, said Thai police official Sommart Meungmuti.
The accident, which left another 12 people injured, is suspected to have been caused by a tyre explosion, he added.
Thailand is usually home to more than two million migrant workers, according to activists.
In the past the authorities turned a blind eye to the presence of illegal labourers because they were needed when the economy was booming.
But now Thailand is on the verge of recession after the economy contracted 2.1% quarter-on-quarter in the first three months of 2014.
The army has floated the idea of creating special economic zones in border areas to better manage the movement of migrant workers, although so far details of the plan remain vague.
The coup followed years of political divisions between a military-backed royalist establishment and the family of fugitive former Thai prime minister Thaksin Shinawatra -- a close ally of Cambodian premier Hun Sen, who once called him an "eternal friend".