This is the first year that the government has exempted immigration fees for migrant workers, allowing them to exit and re-enter Thailand without paying fees.

They can apply for the privilege if they return to Thailand between April 5 to April 30.

The crowds started to queue up at the Aranyaprathet checkpoint in Sa Kaew province before 8am, which is the official opening hour.

They have to present passports and a working slip to immigration officials before being allowed to enter Cambodia.

Extra officials were deployed to help handle the thousands of the Khmer migrants.

Meanwhile Sa Kaew police patrolled along the border to prevent illegal migrants sneaking back to Cambodia.

They arrested six Cambodians after they were unable to present their travel documents.