A vast nation endowed with an abundance of uranium, gold, coal and oil but among the poorest on the planet, Niger is electing a head of state, as well as a new parliament, with Issoufou hoping for a second five-year term.
A total of 7.5 million people are eligible to vote at 25,000 polling stations across the country on the edge of the Sahara Desert. The results are expected within five days.
"I'm waiting. It's 8:00 am and there's nobody. I don't see the number of the polling station and the electoral lists have not been posted," said Ali Issaka, 45, who was waiting to cast his vote at a school in Lazare-1 district in the capital.
In an interview with AFP on Thursday, Issoufou said he was "absolutely" confident of victory and predicted a second-round run-off vote would not be needed.
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Issoufou said he had met his pledges on boosting growth and infrastructure, while shoring up security in the face of attacks by jihadists from neighbouring Nigeria, Mali and Libya.
Should he fail to win a first-round victory, his rivals, who have accused him of planning to rig the result, have agreed to unite behind whoever scores highest amongst them for the second round.
Security was tight with security forces on patrol across the country, the south-east of which has been plagued by attacks by Boko Haram jihadists from neighbouring Nigeria.
"There is no such thing as zero risk but we are are working to uphold security on election day," Interior Minister Hassoumi Massaoudou said ahead of the ballot.
Amadou, a former premier and parliament speaker, heads the Nigerien Democratic Movement (NDM) whose members were tear- gassed by police earlier this month after gathering in their thousands to support their man, known as "the Phoenix" for his ability to rise from the ashes.