Regional leaders are meeting in Abuja to discuss the coup in an emergency meeting of the 15-member Economic Community of West African States.
ECOWAS said it would work with the international community to “do everything to defend democracy and ensure democratic governance continues to take firm root in the region,” Nigerian President and ECOWAS chairman Bola Tinubu said on Friday.
The EU and France have both cut off aid to Niger after presidential guard chief Gen Abdourahamane Tiani declared himself the new leader on Friday, two days after his forces confined the democratically-elected President Mohamed Bazoum to his official residence.
“In addition to the immediate cessation of budget support, all co-operation actions in the domain of security are suspended indefinitely with immediate effect,” EU foreign policy chief Josep Borrell said on Saturday.
The French Foreign Ministry said France had suspended all development aid and budget support with immediate effect. It also demanded Mr Bazoum be reinstated “immediately”.
Thousands of pro-junta protesters gathered outside the French embassy in Niger on Sunday, AFP reported.
Some tried to enter the building and stamped on a plaque bearing the words “Embassy of France in Niger”, replacing it with Niger and Russian flags.
Others shouted “long live Russia”, “long live Putin” and “down with France”, it said.
French development aid for Niger was at around 120 million euros ($132 million) in 2022, and was expected to be slightly higher this year.
Mr Borrell also said the EU would support future efforts to impose sanctions headed by West African blocs.
Mr Bazoum “remains the only legitimate president of Niger”, the EU said, calling for his immediate release.
The eight-member West African Economic and Monetary Union is also meeting in Abuja and could suspend Niger from its institutions, cutting off the country from the regional central bank.
Chad, which borders Niger and is a member of neither organisation, has been invited to the talks.
The African Union has also hit out at Gen Tiani, condemning the coup “in the strongest terms” and ordering the military to restore “constitutional authority” in Niger within two weeks.
Mr Tiani said the takeover was a response to “the degradation of the security situation” in a country plagued by Islamist violence and economic issues.
Mr Tinbu spoke with US Secretary of State Antony Blinken, who warned that Washington's “very significant” aid to Niger is “clearly in jeopardy”.
The US has two military bases in Niger and works closely with authorities to battle Islamist groups in the region.
It also provides hundreds of millions of dollars to the country in security and development aid.
US law on foreign aid prohibits most assistance to any country where the elected head of government has been deposed in a coup or by decree.
Mr Blinken “underscored his support for President Tinubu's continued efforts to restore constitutional order”, Washington said.
The coup “threatens years of years of successful co-operation and hundreds of millions of dollars of assistance”, it added.
On Friday, the US confirmed it was in contact with a “broad array” of military leaders in Niger as the situation evolves.