Amadou, a former premier and parliament speaker, was arrested in November on his return from exile in France and jailed for his alleged role in a baby trafficking scandal.
He left the west African country in August 2014 after losing his parliamentary immunity over the allegations of baby trafficking between Nigeria, Benin and Niger.
He has been denied bail though several other senior political, military and business figures facing accusations in the case have been released pending trial.
Amadou has denied the allegations and denounced the legal process against him as a ploy to keep him out of the elections.
“This is the first time that a presidential candidate has been deprived of his freedom and will not be able to campaign like all the others,” interim Moden president Oumarou Noma told the crowd.
Niger’s constitutional court in January approved 15 candidates for the February 21 presidential polls, including Amadou.
Incumbent Mahamadou Issoufou, elected in 2011, is seeking a second term.
He will also be up against chief opposition leader Seini Oumarou, former president Mahamane Ousmane and ex-planning minister Amadou Boubacar Cisse, among others.
The electoral campaign began on January 30.
Amadou supporters have placarded giant posters of him in the streets of Niamey where Moden vehicles have been crisscrossing the city to drum up support for the jailed contender.