Niger's junta said Ecowas could stage an imminent military intervention in the capital Niamey as the regional bloc was due to hold an "extraordinary summit" on Sunday over the coup in the Sahel state.
Niger's elected president, Mohamed Bazoum, has been held by the military for four days, and General Abdourahamane Tiani, the chief of the powerful presidential guard, has declared himself leader.
Former colonial ruler France and the European Union have suspended security cooperation and financial aid to Niger following the coup, the latest to hit the turbulent Sahel region.
The Economic Community of West African States (ECOWAS) was scheduled to meet in Nigeria's capital Abuja for the summit on Niger, with sanctions a possibility.
In a statement read out on national television on Saturday evening, Niger junta member Amadou Abdramane said the summit's aim was to "approve a plan of aggression against Niger, in the form of an imminent military intervention in Niamey".
The action would be "in cooperation with African countries who are not members of the regional body and certain Western nations", he added.
Thousands of pro-junta protesters gathered outside the French embassy in Niamey on Sunday, with some trying to enter the building, an AFP journalist saw.
Some demonstrators ripped off and stamped on a plaque bearing the words "Embassy of France in Niger", replacing it with Niger and Russian flags, while others shouted "long live Russia", "long live Putin" and "down with France".
ECOWAS has the power to impose sanctions on Niger, which is one of its 15 members.
Last year, its leaders agreed to create a regional security force to intervene against jihadists and prevent military coups.
Details on how that force would work and its funding are still unclear, with Ecowas defence ministers expected to make decisions later this year.
Bola Tinubu, president of Nigeria and Ecowas chairman, said on Friday the West African bloc and the international community "would do everything to defend democracy and ensure democratic governance continues to take firm root in the region".
Ahead of Sunday's gathering, US Secretary of State Antony Blinken spoke to Tinubu to convey his "deep concern" over the situation in Niger, and "underscored his support for President Tinubu's continued efforts to restore constitutional order" there.
Chadian leader Mahamat Idriss Deby Itno, whose country is not an ECOWAS member but borders Niger, has been invited to the summit and arrived in Abuja on Sunday.
Niger's neighbours Mali and Burkina Faso have both experienced military coups since 2020, fuelled by anger at a failure to quash long-running insurgencies by jihadists linked to the Islamic State group and Al-Qaeda.
Tiani said the putsch in Niger was a response to "the degradation of the security situation" linked to jihadist bloodshed, as well as corruption and economic woes.
Turbulent political history
After a wave of condemnation for the coup, punitive measures have already begun.
France -- which has 1,500 soldiers in Niger -- said on Saturday it was suspending development aid and budgetary support to the West African nation, one of the world's poorest countries.
European Union diplomatic chief Josep Borrell meanwhile said the EU would not recognise the putschists, and announced the indefinite suspension of security cooperation with Niger with immediate effect, as well as budgetary aid.
Borrell said the EU was ready to support future decisions taken by Ecowas, "including the adoption of sanctions", echoing a statement by France's foreign minister.
It condemned the coup in "the strongest terms possible" and expressed deep concern over the "alarming resurgence" of military overthrows in Africa.
The United States -- which has about 1,000 troops in Niger -- has offered Bazoum Washington's steadfast support and warned those detaining him that they were "threatening years of successful cooperation and hundreds of millions of dollars of assistance".
Landlocked Niger often ranks last in the United Nations' Human Development Index, despite vast deposits of uranium.
It has had a turbulent political history since gaining independence in 1960, with four coups as well as numerous other attempts -- including two previously against Bazoum.
