The officers 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.
Twelve soldiers appeared on television on Wednesday, announcing they were cancelling the results of the election and dissolving “all the institutions of the republic”.
They also said the country’s borders had been closed “until further notice”.
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”.
Journalists from the Reuters and AFP news agencies reported the sounds of loud gunfire could be heard in the country’s capital, Libreville, following the broadcast.
There was no immediate response by the government to the soldiers’ announcement.
We have decided to defend peace by putting an end to the current regime
Gabonese Army Soldiers
Serious ‘concerns’
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lacked ballot papers bearing his name, while the coalition he represents said the names of some of those who had withdrawn from the presidential race had still been on the ballot sheet.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.
As polls closed, the government announced a curfew and suspension of internet access for security reasons.
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.
