Issoufou -- a key Western ally in the fight against Islamist militants in West Africa -- is widely expected to seek a second five-year term in February 2016 and is the current favorite, with the opposition fragmented and one leader in jail.
"There are failings in the voter lists," said Sabiou Gaya, deputy coordinator for the election committee, adding some of the errors included enrolment mistakes and missing polling booths.
Corrections are expected to be completed by Nov. 22, he said. Officially, the final list was due to be handed to the Independent National Electoral Commission by end-October.
On Saturday, Niger opposition leader Hama Amadou was arrested when he flew into Niamey as security forces blocked off the airport, a year after he fled the country when sought by investigators probing a child trafficking ring.
The former ally of Issoufou was transferred to the Filingue prison overnight, about 160 kilometers (99 miles) northeast of Niamey, security sources said on Sunday. He has denied the charges and claims they are politically motivated.
Another challenger to Issoufou, Abdou Labo, was officially named as a presidential candidate by his Democratic and Social Convention party (CDS) on Saturday.
Labo was also briefly detained for involvement in the trafficking of babies into Niger from neighboring Nigeria last year, an allegation he denies.
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