West African leaders were to meet on Sunday for a crisis summit on the coup in Niger, where protesters tried to storm the French embassy after the junta warned of an "imminent military intervention".
In the third coup to fell a leader in Africa's restive Sahel region in as many years, Niger's elected president, Mohamed Bazoum, has been held by the military since Wednesday.
General Abdourahamane Tiani, the head 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 15-nation Economic Community of West African States (ECOWAS) prepared to gather for an "extraordinary summit" in Nigeria's capital Abuja on Sunday to discuss the crisis.
ECOWAS has the power to impose sanctions on Niger, which is one of its members and is one of the world's poorest nations, often ranking last on the UN's Human Development Index.
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 intervention would be "in cooperation with African countries who are not members of the regional body and certain Western nations", he added.
Thousands of people waving Russian and Niger flags rallied outside the national parliament in Niamey on Sunday in a show of support for the junta.
They then moved on to the French embassy, shouting "long live Putin" and "down with France", before being dispersed by soldiers with tear gas, an AFP journalist saw.
Some tried to storm the embassy, but were dispersed.
Despite a junta ban on protests, a soldier standing in a pick-up truck waved to the crowd, shouting "Russia, Russia, Russia!", "long live Niger's army" and "Tiani, Tiani, Tiani".
A number of demonstrators headed for the embassy of the United States, which has voiced support for efforts by ECOWAS chairman Bola Tinubu, the president of Nigeria, "to restore constitutional order" in Niger.
France condemned the assault on its embassy, warning it would retaliate if its citizens or interests were attacked, and said it would support all regional initiatives to restore order in Niger.
"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.
Last year, ECOWAS leaders agreed to create a regional security force to intervene against jihadists and prevent military coups, but details on the force and its funding are still unclear.
Nigeria's Tinubu 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".
Chadian President Mahamat Idriss Deby Itno, a military ruler whose country is not an ECOWAS member but borders Niger, has been invited to the summit.
Niger's neighbours Mali and Burkina Faso have both been beset by military coups since 2020, fuelled by anger at the civilian authorities' 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.
It called for "an immediate return to constitutional order" and President Bazoum's reinstatement.
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.
The African Union has given the military in Niger two weeks to restore "constitutional authority".
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.
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