Multiple reports suggest Malian President Ibrahim Boubakar Keïta has been arrested by mutinying soldiers.
Their actions began with gunfire inside a key military base, some 15km (nine miles) from the capital, Bamako, on Tuesday morning.
In the centre of Bamako, young men looted a government-owned building and set it on fire.
The mutiny has been condemned by regional group ECOWAS and former colonial power France.
The unrest coincides with calls for more protests to demand that the president resign.
Mr Keita won a second term in elections in 2018, but there is widespread anger over corruption, the mismanagement of the economy and the worsening security situation with jihadist and communal violence on the increase.
It is not yet clear how many soldiers have taken part in the mutiny. Some reports say it was fuelled by a pay dispute.
The Economic Community of West African States (ECOWAS) urged the mutineers to return to barracks.
“This mutiny comes at a time when, for several months now, ECOWAS has been taking initiatives and conducting mediation efforts with all the Malian parties,” its statement said.