Niger's opposition parties say they do not recognize the first partial results of the recent presidential election, citing electoral fraud.

"We cannot recognize them such as they have been proclaimed so far," said Aboubacar Cisse, spokesman for the opposition coalition, on Tuesday.

The opposition has accused the government of tampering with the ballots.

Prior to the poll, the opposition had anticipated a run-off vote, vowing there would be no easy victory for incumbent Mahamadou Issoufou.

"There will be a second round," said a spokesman for former premier Seini Oumarou, one of the leading opposition candidates.

But Issoufou, 63, is confident of winning a second five-year term.

In an interview with AFP last week, Issoufou, 63, said he was "absolutely" confident of a swift victory in the first round of the poll.

Earlier, Interior Minister Hassoumi Massaoudou dismissed allegations of vote-rigging.

"The elections are free and transparent … They're talking nonsense. We're going to win a legitimate victory." Massaoudou said.

Voting wrapped up on Monday after it was extended for a second day after several cities did not receive their ballot papers on Sunday.

Final results of the vote are expected to be announced in around five days, officials said.

Despite an abundance of uranium, coal and oil, Niger is one of the poorest countries in the world.

A total of 7.5 million people were eligible to vote across the country on the edge of the Sahara desert, where security is a growing concern after attacks by militants from neighboring Nigeria, Mali and Libya.