More than 120,000 Cambodians leave Thailand
The number of Cambodians leaving Thailand has soared to more than 120,000 in recent days, officials say.
Many fear Thailand's new military rulers are about to crack down on migrant workers, which Bangkok denies.
Cambodian authorities are now reportedly preparing to deal with the influx.
The new figure is a significant proportion of the number of Cambodians in Thailand, estimated to be at least 150,000.
A huge logistics and assistance operation is now under way to help the workers get back to their hometowns and villages.
At the border with Cambodia, trains are arriving from Bangkok packed with Cambodians, says the BBC's Jonathan Head in the border town of Aranyaprathet.
The workers are being transferred by the Thai army to trucks which take them over the border, he says.
The town of Poipet on the Cambodian side of the border is now filled with returnees.
Cambodia, meanwhile, is scrambling to accommodate the sudden influx of returning workers.
The AFP news agency reported that almost 300 cars and military trucks have been arranged to transport workers away from Poipet.
Cambodia's labour minister Ith Samheng has said the government is working on a jobs creation programme.
Many of those leaving are worried by warnings from the Thai government that it will take action on illegal immigrants.
There are also rumours that Cambodians will be targeted, whether they are legal or illegal, says our correspondent.
The Thai authorities have promised that only illegal immigrants are being singled out, and that documented workers are still welcome.
Cambodians are often viewed as suspect because of support their government has given to the controversial former Prime Minister Thaksin Shinawatra.
Mr Thaksin was ousted in a coup in 2006, and his sister Yingluck was similarly unseated as prime minister by the military last month.
Thai industry depends on the two to three million migrants who have come from neighbouring countries.
It is unclear whether some of the migrants are being forced to leave.
Wire reports have quoted some saying they were told to to leave by Thai soldiers, and a Cambodian human rights body has accused the Thai military of "forcefully" expelling migrants.
But other migrants have also been quoted as saying they decided on their own to return to Cambodia.