He was detained on 24 July with no reason given to his colleagues at the SJU.
"We lost contact with the President of SJU since Wednesday afternoon, July 24TH, 2019 and later it became clear that he was detained but we did not have any information on the reason for his arrest", the SJU said in a statement.
The IFJ, SJU and FAJ had repeatedly called on Sudan's ruling Transitional Military Council to "immediately release" Sadiq, prominent editor of Al-Sayha newspaper and international union leader, or that he be put on trial with every legal guarantee.
IFJ General Secretary, Anthony Bellanger, said: "We welcome the release of our comrade Sadiq Ibrahim Ahmed but strongly condemn the circumstances in which he was arrested. Being five days under arrest and incommunicado without any explanation is unacceptable. We urge the Sudanese Military and other Sudanese relevant authorities to immediately explain the reasons for his arrest".