The head of Niger's national electoral commission (CENI) on Thursday said the country's presidential run-off vote will go ahead as planned, even after the opposition declared that its candidate was withdrawing from the race.

"We are determined to organise these polls on March 20," CENI chief Boube Ibrahim told reporters in Niamey, adding that the commission had to respect "constitutional deadlines".

Niger's opposition coalition on Tuesday said its candidate Hama Amadou, who has campaigned from behind bars and came second in the first round, was dropping out of the "unfair" race.