And they imposed financial sanctions on the military, after the latest coup in the Sahel region raised alarm on the continent and in the West.
Niger's elected President and western ally, Mohamed Bazoum, has been held by the military since Wednesday, marking the third coup in as many years in the Sahel.
The head of the powerful presidential guard, Gen Abdourahamane Tiani, has declared himself leader.
At an emergency summit in Nigeria, the Economic Community of West African States regional bloc demanded Mr Bazoum be reinstated, within a week.
Otherwise, the bloc said it would take "all measures" to restore constitutional order.
"Such measures may include the use of force for this effect," it said in a statement, adding that Ecowas defence chiefs were to meet on Sunday.
"No more time for us to send a warning signal... It's time for action," said Bola Tinubu, president of Nigeria and Ecowas chairman.
It was not immediately clear how the 15-member alliance could use force.
Last year, the bloc agreed to create a regional security force to intervene against extremist militias and prevent military coups, but details on the force and its funding have not been given.
The bloc also placed financial sanctions on the junta leaders and on the country, freezing "all commercial and financial transactions" between member states and Niger – one of the world's poorest nations, often ranking last on the UN's Human Development Index.
Niger's Prime Minister Ouhoumoudou Mahamadou told broadcaster France24 on Sunday that sanctions were "going to be a disaster", economically and socially.
On Saturday, the junta condemned the Ecowas summit, saying its aim was to "approve a plan of aggression against Niger, in the form of an imminent military intervention in Niamey".
The intervention would be "in co-operation with African countries who are not members of the regional body and certain western nations", junta member Amadou Abdramane said on national television.
Niger soldiers declare coup on national TV - video
Former Niger president Mahamadou Issoufou, who Mr Bazoum succeeded as head of state, on Sunday said he intended to negotiate with the junta to restore his successor to the presidency.
"I have undertaken, by various ways, to find a negotiated solution that will allow to free President Mohamed Bazoum and to reinstate him," Mr Issoufou wrote on Twitter.
In the capital Niamey, thousands of people waving Russian and Niger flags rallied outside the national parliament in a show of support for the junta.
They then moved to the French embassy, shouting "long live Putin" and "down with France".
Some tried to storm the embassy but were dispersed with tear gas, AFP reported.
France condemned the assault on its embassy, warning it would retaliate if its citizens or interests were attacked and said it had strengthened security at the embassy.
"Should anyone attack French nationals, the army, diplomats and French interests, they will see France respond in an immediate and intractable manner," the French presidency said.
There are between 500 and 600 French nationals and 1,500 troops in Niger, but France's Foreign Minister Catherine Colonna told RTL radio that while they have been contacted and "precautionary measures have been taken", no evacuation order was in place.
Niger's neighbours and fellow former French colonies Mali and Burkina Faso had military coups in 2020, fuelled by anger at the civilian authorities' failure to quash long-running insurgencies by militants linked to ISIS and Al Qaeda.
Mr Tiani said the takeover in Niger was a response to "the degradation of the security situation" linked to miliant bloodshed, as well as corruption and economic woes.
After condemnation for the coup, punitive measures have already begun in the West.
France said on Saturday it was suspending development aid and budgetary support to the West African nation.
EU foreign affairs chief Josep Borrell, meanwhile, said the EU would not recognise the junta, and announced the indefinite suspension of aid and security co-operation with Niger with immediate effect.
The US, which has about 1,000 troops in Niger, has offered Mr Bazoum Washington's steadfast support and warned those detaining him that they were "threatening years of successful co-operation and hundreds of millions of dollars of assistance".
Britain on Sunday said it condemned attempts to undermine democracy, peace and stability in Niger and said the UK stood by the Ecowas in its efforts to restore stability in Niger.
"The UK is a committed partner of Niger's democratically elected government and calls for President Bazoum to be immediately reinstated to restore constitutional order," its statement read.
The African Union condemned the coup and expressed deep concern over the "alarming resurgence" of military overthrows in Africa.
Niger has had a turbulent political history since gaining independence in 1960, with four coups and many other attempts, including two others against Mr Bazoum.