This as President Mohamed Bazoum is confined at his residence by members of his presidential guard.
Late in the evening, their leaders, calling themselves the Defence and Security Forces (FDS), declared they had "decided to put an end to the regime," and all institutions were being suspended, the borders closed and a night-time curfew imposed.
Armed Forces chief General Abdou Sidikou Issa on Thursday swung his weight behind the putschists.
"The military command... has decided to subscribe to the declaration made by the Defence and Security Forces... in order to avoid a deadly confrontation," he said in a statement.
Earlier, Bazoum defiantly stood his ground as the condemnation of the putsch swelled from African and international organisations and allies France and the United States.
Several hundred people in Niamey and a similar number in the town of Dosso took part in shows of support for the coup leaders, AFP journalists saw.
In scenes redolent of what happened in Mali following a coup there in 2020, some held Russian flags and chanted anti-French and pro-Russian slogans.
"France was unable to take care of our problems - we need to take our destiny in hand," said their leader, who gave his name as Issouf and said he worked in a garage.
Youths from the gathering in Niamey went to the headquarters of Bazoum's PNDS party and ransacked it, setting fire to cars.
The coup leader appeared on national television urging "the population to remain calm", after the violence.