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.
Twelve soldiers appeared on television early on Wednesday morning, announcing they were cancelling the results of the election and dissolving “all the institutions of the republic”.
They added that the country’s borders had been closed “until further notice”.
If confirmed, this would be the eighth coup in former French colonies in Africa in the past three years.
However, most of the others have been further north, in the Sahel region where an Islamist insurgency has led to rising complaints that the democratically elected governments were failing to protect the civilian populations.
French Prime Minister Elisabeth Borne said her country was following the situation closely, while the European Union’s foreign policy chief said a military takeover would increase instability in Africa.
“This is a big issue for Europe,” said Josep Borrell.
Meanwhile, French mining group Eramet, which employs thousands of people in Gabon, said it had stopped all work in the country for security reasons.
The soldiers who announced the apparent coup said they were from the Committee of Transition and the Restoration of Institutions and represent security and defence forces in the country.
One of the soldiers said on TV channel Gabon 24: “We have decided to defend peace by putting an end to the current regime.”
This, he added, was down to “irresponsible, unpredictable governance resulting in a continuing deterioration in social cohesion th,at risks leading the country into chaos”.
The sounds of loud gunfire could be heard in the country’s capital, Libreville, following the broadcast. (BBC)