The assailants, from a nearby air base, fled after the presidential guard met their attack with heavy shelling and gunfire, three security sources said, requesting anonymity because they were not authorised to speak to media.
Government spokesman Abdourahamane Zakaria said several people had been arrested while others were still being sought, but that the situation was under control.
“The government condemns this cowardly and retrogressive act that aims to endanger democracy and the rule of law to which our country is resolutely committed,” he told a news conference.
Assuring people they could go about their daily lives, he said: “The government congratulates the presidential guard and the other defense and security forces for their prompt reaction, a testament of their loyalty to the republic.” He did not take questions.
President Mahamadou Issoufou is stepping down after two five-year terms and president-elect Mohamed Bazoum, the ruling party candidate, is due to be sworn in on Friday after an election victory disputed by his opponent Mahamane Ousmane.
Former U.S. Sahel envoy J. Peter Pham earlier tweeted that both the president and president-elect were safe, and the president’s office shared photos on Twitter of Issoufou presiding over the swearing-in ceremony for two top judges.
Ousmane’s whereabouts were not known.
There have been growing attacks by Islamist militants as well as protests in the country following Bazoum’s victory in a February presidential election runoff. Ousmane, a former president who lost that contest, has rejected the results and said there was fraud.
In pockets of the capital on Wednesday, Ousmane supporters took to the streets for a scheduled protest and clashed with police, who fired teargas to disperse them, according to witnesses, who also said roads out of the city had been closed.
Bazoum’s election is the first democratic transition of power in a state that has witnessed four military coups since independence from France in 1960, including one which toppled Ousmane in 1996.
The heavy gunfire started around 3 a.m. local time (0200 GMT) and lasted for around 30 minutes, according to a Reuters witness. By 10 a.m. traffic had resumed in the area and the situation appeared normal, said several witnesses.
The U.S. Embassy in Niamey said it was closed for the day due to gunshots heard in the neighbourhood and warned that the security situation remained fluid in the post-election period.
Rising insecurity in the region caused by jihadists linked to al Qaeda and Islamic State has compounded economic challenges for Niger including drought, the COVID-19 pandemic, and low prices for its top export uranium.
A coup in neighbouring Mali in August last year overthrew president Boubacar Keita. Under pressure from regional states, the junta ceded power to a transitional government that will govern until elections next year.
