Army officers have appeared on national television in Gabon to say they have taken power.
They said they were annulling the results of Saturday’s election, in which President Ali Bongo was declared the winner.
The electoral commission said Mr Bongo had won just under two-thirds of the votes in an election the opposition argued was fraudulent.
His overthrow would end his family’s 53-year hold on power in Gabon.
Gabon is one of Africa’s major oil producers, while nearly 90% of the country is covered by forests.
It joined the Commonwealth in June 2022, becoming one of its few members not to have been a British colony.