Aid agencies and human rights groups working in eastern Democratic Republic of Congo say they are being targeted by local militias.
The rebels were supposed to join the national army
Human rights group Amnesty International says several leading activists had gone into hiding after receiving death threats.
It suggested local rebel groups were behind the "concerted campaign".
DR Congo is recovering from a five-year war but gangs of gunmen still roam freely across much of the east.
Former rebels were brought into a power-sharing government, but Vice-President Jean Pierre Bemba's group has threatened to pull out by the end of this month if elections scheduled for June are postponed.
On Wednesday, the government said that two TV channels and a radio station belonging to Mr Bemba had been taken off-air because they had carried an interview which had insulted President Joseph Kabila.
South African President Thabo Mbeki is due in the capital, Kinshasa, next week to try and keep the peace process on track.
Last month, there was fighting in North Kivu province between a former rebel group, which had joined the government, and the army.
Aid agency Medecins Sans Frontieres - Holland agreed with Amnesty about the dangers of working in North Kivu.
It said that it had been forced to close a base after being attacked by armed men who stole money, equipment and a car.