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.
The soldiers 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 that risks leading the country into chaos".
The sounds of loud gunfire could be heard in the country's capital, Libreville, following the broadcast.
BBC World Service Africa editor Will Ross says this suggests the coup is perhaps not "done and dusted" and there will be some resistance.
There was no immediate response by the government to the soldiers' announcement and the whereabouts of Mr Bongo are unknown.
Internet access was suspended following Saturday's election for security reasons, however this was restored shortly after the apparent takeover. A curfew is also in place.
As in previous general elections in Gabon, there were serious concerns about the process in Saturday's vote.
Main opposition candidate Albert Ondo Ossa complained that many polling stations lacked ballot papers bearing his name, while the coalition he represents said the names of some of those who had withdrawn from the presidential race had still been on the ballot sheet.
Campaign group Reporters Without Borders said foreign media had been banned from setting foot in the country to cover the vote.
Both of Mr Bongo's previous wins were disputed as fraudulent by opponents. This time, controversial changes were made to voting papers just weeks before election day.
Mr Bongo came to power when his father Omar died in 2009.
In 2018, he suffered a stroke which sidelined him for almost a year and led to calls for him to step aside.
The following year, a failed coup attempt saw mutinying soldiers sent to prison.