Niger is due to hold a controversial run-off election amid opposition calls for a boycott alleging fraud.
President Muhamadou Issoufou is expected to win a second term in the vote against former prime-minister and parliamentary speaker Hama Amadou.
Mr Amadou was jailed in November on baby-trafficking charges, which he denies. He was flown to Paris last week for medical treatment.
One of the world’s poorest countries, Niger faces a growing jihadist threat.
The first round of voting in February, which happened when Mr Amadou was in prison, was marred by delays and voting had to be extended to the next day in some places.
Mr Issoufou got 48% of the votes while Mr Amadou came second with 17%.
The opposition coalition called for the boycott citing rigging in the first round and what they called persistent unfair treatment of their candidate by the authorities.
Mr Amadou is accused of child-trafficking from neighbouring Nigeria. He and about 30 other people are facing trial on alleged purchase of babies.
He denies the allegations, and his supporters say his detention is politically motivated. The government, however, says it is purely a judicial matter.
His medical condition is unclear, but his health reportedly deteriorated in prison.
In a separate development, the rapporteur of the electoral commission suspended his involvement in the vote, alleging violations of electoral law by other members of his team.
Maikoul Zodi, who was the only representative of civil society on the commission, said the irregularities included steps to prevent the opposition from monitoring the ballot.
Niger at a glance
- Population is of 16.6 million people; 7.5 million are eligible voters
- UN report said last month about two million people would need urgent food aid this year due to drought
- Challenges include unemployment and absence of infrastructure
- Country has oil, gold and uranium deposits, as well as agricultural potential
Last December, the president said the country’s security forces had foiled another attempted coup, prompting arrests of some military and civilian figures.
Niger faces threats from Boko Haram militants based in neighbouring Nigeria, and there are also security concerns on its borders with Libya, Mali and Algeria.
On Thursday, militants from al-Qaeda and Boko Haram killed four members of the security forces in two separate attacks.