Bolivian President Luis Arce has demanded democracy be “respected” after soldiers were reported to have moved into central La Paz.
Unverified videos showed armoured vehicles and soldiers in Murillo Square, in the centre of the capital La Paz.
The presidential palace was rammed by an armoured vehicle, with soldiers reportedly storming the interior.
Former President Evo Morales warned of an imminent “coup,” accusing General Juan José Zúñiga of planning to seize power and calling on supporters to strike and block roads.
The El Deber newspaper quoted Gen Zúñiga as telling reporters from Murillo Square after it was taken by troops: “We are going to recover this homeland.”
But President Arce denounced what he called “irregular” troop movements, saying “democracy must be respected”.
In more pointed language, Mr Morales wrote: “A group of the Challapata Special Regiment ‘Mendez Arcos’ took over Plaza Murillo with snipers.
“This seems to indicate that they prepared the coup d’état in advance.
“I ask democratically-minded people to defend the homeland from certain military groups that act against democracy and the people.”
The Reuters news agency quoted Gen Zúñiga as saying: “There will be a new cabinet, surely things will change, but our country cannot continue like this any longer.”
Mr Morales has split from Mr Arce, although they are both members of the same socialist movement.