As he was put on a plane to neighbouring Cameroon on Monday, Dietrich told reporters he had been mistreated by police.
Chadian opposition members said he had been detained by the National Security Agency.
Police said his visa application had been denied in Paris and he had gone to to the Cameroonian capital Yaoundé to file a new one.
The writer announced his visit on Facebook before leaving, laying into “the dictatorship that oppresses Chad” and accusing Déby’s government of human rights violations.
Last January he published Les enfants de Toumai (“The Children of Toumai”), a love story about a Chadian Muslim girl and a Maoist student from a Christian background.
Chad is waiting for results from 10 April’s presidential election, in which a dozen candidates tried to end Déby’s 26-year rule.
Source: rfi afrique