About 12,000 Nigerians are being repatriated from neighbouring Cameroon, Nigeria’s National Emergency Management Agency (Nema) has said.
Some of those returning via the Sahuda border post have told the BBC they are being sent home against their will.
Nema says most of them had sought refuge in Cameroon following attacks by Boko Haram Islamist militants.
There have been a spate of recent attacks in northern Cameroon blamed on Boko Haram.
The authorities in Cameroon believe the militants are entering the country disguised as refugees.
Some of the Nigerians being returned are believed to be those who were rounded up for not having the correct documents.
So far 1,121 people have come through the Sahuda border post in Adamawa State and the others are waiting to go through immigration and security screening, Nema said.
Boko Haram has vowed to attack Cameroon because the country is supporting the Nigerian military’s mission to defeat them – and is part of a newly boosted multi-national force to battle the militants.