Niger closed its airspace due to the "threat of intervention" as the junta defied a deadline from the West African bloc Ecowas to reinstate democratically elected President Mohamed Bazoum or face possible military action.
Ecowas issued its ultimatum a week ago, demanding the generals relinquish power by midnight Sunday (2300 GMT).
"Faced with the threat of intervention, which is becoming clearer through the preparation of neighbouring countries, Niger's airspace is closed from this day on Sunday... for all aircraft until further notice," the junta said in a statement released shortly before the deadline passed.
Any attempt to violate the country's airspace would meet with an "energetic and immediate response", the statement added.
Early Monday, there were no aircraft operating in Niger's skies, according to the flight tracking website Flightradar24.
In a separate statement, the now-ruling National Council for the Safeguard of the Homeland (CNSP) said there had been a "pre-deployment in preparation for intervention" made by two Central African countries, without naming them.
"Any state involved will be considered co-belligerent," it warned.
At the 30,000-seat Seyni Kountche stadium, named after Niger's first coup d'etat leader in 1974, CNSP leaders, including General Mohamed Toumba, greeted a jubilant crowd.
Ecowas military chiefs of staff agreed Friday on a plan for a possible intervention to respond to the crisis, the latest of several coups to hit Africa's Sahel region since 2020.
"We want diplomacy to work, and we want this message clearly transmitted to them (the military) that we are giving them every opportunity to reverse what they have done," ECOWAS commissioner Abdel-Fatau Musah said.
But he warned that "all the elements that will go into any eventual intervention have been worked out", including how and when force would be deployed.
Former colonial power France, with which Niger's new rulers broke military ties after taking power, said it would "firmly" back whatever course of action Ecowas took after the deadline expired.
Ivory Coast President Alassane Ouattara called on the coup leaders to stand down late Sunday on the eve of his own country's Independence Day.
"We condemn the attempted coup in Niger, which poses a serious threat to peace and security in the sub-region," Ouattara said, adding it was "essential" to "constitutional order" that Bazoum be allowed to govern.
In the dusty alleyways of Niamey's Boukoki neighbourhood, residents were defiant at the prospect of an armed intervention by Ecowas.
"We're going to fight for this revolution. We're not going to retreat faced with the enemy, we're determined," said Boukoki resident Adama Oumarou, adding that "we were waiting for this coup for a long time".
Algeria, which shares a long land border with Niger, has warned against a military solution.
"We categorically refuse any military intervention," Algerian President Abdelmadjid Tebboune said in a television interview Saturday, saying it would be "a direct threat to Algeria" as it shares an almost 1,000-km-long border with Niger.
On Sunday, France said it was cutting off development aid and budgetary assistance to Burkina Faso, which alongside Mali has said it would consider any attack on Niger as a "declaration of war".
Niger has played a key part in Western strategies to combat jihadist insurgencies that have plagued the Sahel since 2012, with France and the United States stationing around 1,500 and 1,000 troops in the country, respectively.
France has already evacuated hundreds of its citizens from Niger since the coup, and on Sunday, Italy's defence ministry said it had flown 65 military personnel from Niger, along with 10 US military personnel.
Anti-French sentiment in the region is on the rise, while Russian activity, often through the Wagner mercenary group, has grown. Moscow has warned against armed intervention from outside Niger.
Niger, one of the poorest countries in the world, relies heavily on foreign aid that could be pulled if Bazoum is not reinstated as head of state, Paris has warned.
Bazoum, 63, has been held by the coup leaders with his family in his official Niamey residence since July 26.
He won an election in 2021 that ushered in Niger's first-ever transfer of power from one civilian government to another.
Nigeria has already cut electricity supplies to its neighbour Niger, raising fears for the humanitarian situation, while Niamey has closed the vast Sahel country's borders, complicating food deliveries.
Senior Nigerian politicians have urged President Bola Tinubu to reconsider the threatened military intervention.
