Logistical problems in some areas prevented polling the previous day, stretching the presidential and legislative elections in Niger into a second day on Monday.
"The vote restarted on Monday in areas where the polling stations didn't work yesterday," said Kadi Moustapha, a spokesman for the West African Network for Edification and Peace observer group.
Observers said that polls will be open in four of the eight regions: the northeastern Tahoua region, and Zinder, Diffa and Tillaberi, in the east, southeast and west respectively.
Diffa is under a state of emergency after militant group Boko Haram, based in neighbouring Nigeria, staged a series of attacks there in recent months.
Former prime ministers Seyni Oumarou and Hama Amadou are President Mahamadou Issoufou’s strongest opponents. Issoufou is supported in this election for his success in preventing militants from setting up a base in their country and reducing poverty.
Critics, however, find Issoufou repressive because of the detention of former prime minister, current opponent Hama Amadou, who is accused of involvement in a child-trafficking case and was jailed in November last year.
The government says such criticism is politically motivated. Recent detentions of politicians, journalists and a singer who released a song critical of Issoufou are shown as evidence by critics.