Officials from the Democratic Republic of Congo say 40,000 illegal immigrants have been driven across the border from neighboring Congo in the past few weeks.
Authorities in Congo Brazzaville launched an operation called "The Slap of the Elders" earlier this month to expel the illegal immigrants.
It is part of a government crackdown on illegal workers who frequently cross the river between Brazzaville and Kinshasa, the DRC capital.
Many of those who have been sent back to Congo Kinshasa complain of brutal treatment by Congo Brazzaville police.
Brazzaville mayor Hugues Ngouelondele has apologized to anyone who has been mistreated and says guilty police officers will be punished.