Supporters of Niger's opposition leader Mahamane Ousmane set up burning barricades and clashed with riot police in the capital Niamey on Wednesday, as the former president declared himself the winner of Sunday's vote, despite losing the official tally.

On Tuesday the election commission declared ruling party candidate Mohamed Bazoum winner with 55.75% 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% against Bazoum's 40.7% 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.

The Timia district in the city of Agadez reported a participation rate of 103%, he said, returning 99% in favour of Bazoum. Ousmane alleged his teams there had been forced to sign result certifications at gunpoint.

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