Niger's opposition presidential candidate Hama Amadou was taken from his Niamey jail cell and flown to a Paris hospital Wednesday, four days before a runoff election.
Amadou has been in jail since November on baby-trafficking charges — allegations he and his supporters say are politically motivated.
Opposition officials say Amadou has been sick since he was jailed, but the nature of his illness is unclear.
Amadou, a former parliament speaker, plans to have his name on the ballot Sunday, even though his opposition coalition said last week it would boycott the runoff.
It said results of the first round, won by incumbent President Mahamadou Issoufou, were "grotesque and cooked up."
The coalition also accused Niger's top court of conspiring with the government to ensure Issoufou's re-election. The opposition alleges Issoufou is becoming increasingly authoritarian.
Despite a wealth of energy resources, Niger is one of the world's poorest countries.