Niger's President Mahamadou Issoufou has failed to secure an outright majority in Sunday's elections, taking the contest into a second round.
Mr Issoufou took 48% of the vote, with jailed opposition leader Hama Amadou on 18%, official preliminary results say.
The run-off vote between the two candidates is scheduled for 20 March.
Mr Amadou is currently behind bars accused of trafficking babies, a charge he strongly denies.
President Issoufou is running for a second term.
The run-up to the vote was marred by accusations of repression and a row over identification documents.
The authorities announced that roughly 1.5 million people without ID papers would be able to cast their ballots by having witnesses vouch for them, in a move that was condemned by opposition leaders.
Niger is rich in natural resources, including uranium and oil, but is one of the poorest countries on Earth, ranking last in the UN Human Development Index.
Niger is seen as an important ally of Western powers in the fight against militant Islamists in the fragile Sahara region.
However, the country is far from stable. Corruption, food shortages and porous borders remain serious problems.