Hama Amadou, a major opposition figure approved to stand in Niger's February elections just two days ago, failed on Monday in an attempt to gain release from prison in a plea to the country's appeals court.
President Mahamadou Issoufou, a key Western ally in the fight against Islamist militancy in the fragile Sahara region, is expected to win a second term; but critics accuse him of becoming increasingly authoritarian, especially after uncovering what he described as an aborted coup in December.
Amadou, a one-time ally to the president, was jailed in November for alleged complicity in a baby trafficking ring upon return from a year-long exile. He has publicly denied the charges, which he says are politically motivated.
The opposition figure was among 15 candidates that Niger's constitutional court approved on Saturday to stand in presidential elections in the uranium-producing West African country on February 21.
"With this rejection of conditional release, the regime succeeded with its first knock-out blow," said Souley Oumara, his lawyer. The appeals court did not give a reason for its decision, and Amadou can appeal to the supreme court. The government was not immediately available for comment.
The ruling could reignite tensions between government and the opposition which had been temporarily allayed by the implementation of changes to the electoral register sought by Issoufou's critics.
"There's no reason to keep Hama [Amadou] in prison, especially since he is not directly accused in this alleged child [trafficking] affair and when all the other defendants have been released," said opposition spokesman Ousseini Salatou.
Ali Sadou, a supporter of Amadou's Moden party in the streets of Niamey, said: "The government must have put pressure on the judges because they are afraid of our leader."
Amadou's lawyers were not present in court on Monday because Niger's bar association called a one-day strike against what it sees as arbitrary detentions by the president's secret service that have denied lawyers access to clients.
The country's secret service, the General Directorate of the Documentation and Security of the State, had interrogated Amadou, as well as members of his party and other opposition figures.
"The DGDSE (secret service) cannot arrest and deny the right to a defense," said Samna Daouda, the head of Niger's bar association. "We must defend liberty and rights."
Issoufou said in a televised address in December that plotters had planned the use of aerial firepower and had prevented transport of military vehicles from the capital to southern region Diffa. Several military officials were arrested. Amadou's imprisonment was not in connection with the alleged coup attempt.