Security forces have surrounded the Ouaddaï Royal Palace in Chad's eastern town of Abéché to prevent possible demonstrations after President Idriss Déby's appointment of a new sultan.
The opposition accuses the government of interfering in tradition as the new sultan, Chérif Abdelhadi Mahdi, is not related to his predecessor.
The former ruler of Ouaddaï was among six sultans suspended by the president in May for incompetence.
He was later dismissed in July following ethnic clashes in the troubled region that left 46 people dead and more than 20 injured.