The election commission on Tuesday declared ruling party candidate Mohamed Bazoum winner with 55.75 per cent of the ballots cast, sparking overnight street demonstrations that continued into Wednesday.

Police were seen beating protesters with fists and batons, in streets strewn with rocks and abandoned goods carts. Elsewhere a group of young men walked down a road with a picture of Ousmane, waving sticks and threatening passers-by.

The election was meant to usher in Niger's first transition from one democratically elected leader to another, with President Mahamadou Issoufou stepping down after two five-year terms.

Ousmane rejected the results in several areas where he said "irregularities" had occurred, claiming he had won 50.3 per cent against Bazoum's 40.7 per cent in the presidential run-off.

"Thus, we reject the results in their entirety wherever irregularities have been noted," he said in a video address from Zinder, the country's second largest city.

The internet was severely restricted on Wednesday, forcing Ousmane's campaign team to distribute the video to journalists in Niamey by hand.

"We reserve the right to exercise our right to appeal for annulment so that justice can be done," said Ousmane, a former president who was toppled by a coup in 1996.

An observer mission from the Economic Community of West African States said the vote was held "under free, fair, credible and transparent conditions".

Australian Associated Press