One of the wives of Hama Amadou, the main rival of Niger President Mahamadou Issoufou, is among the 17 people including 12 women being held in the case since the end of June.
Abdou Labo "was taken to Niamey's Say prison after he was heard by a judge who remanded him in custody", one of the minister's aides told AFP.
All suspects have been charged with substituting children, forgery and making false statements.
Private television channels showed Labo's car as it left the Niamey courthouse under police escort.
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Labo, who was interior minister between 2011 and 2013, is deputy leader of the opposition Democratic and Social Convention headed by Mahamane Ousmane, the country's first democratically elected president between 1993 and 1996.
He entered the government in spite of his party's opposition.
"In this alleged child trafficking case no evidence has been provided to prove it," Amadou told Radio France internationale (RFI) at the end of July. "This case is political."
But Justice Minister Marou Amadou warned that "closing our eyes would have been political.
Public prosecutors said there was "evidence" that the law was violated after a thorough investigation in the three countries.