The announcement came minutes after the country's election body announced that President Ali Bongo had won a third term in the polls.
The officers said on television channel Gabon 24 that they represented all Gabonese security and defense forces. They said the election results were cancelled, all borders were closed until further notice and state institutions were dissolved.
Gunfire could be heard in the capital Libreville after the statement announcing the ousting of Bongo, whose family have ruled the country that produces oil and manganese for more than half a century, a Reuters reporter said.
There was no immediate comment from the government of Gabon, which is a member of the oil producer group OPEC, and there were no immediate reports on the whereabouts of Bongo, who was last seen in public casting his vote in the vote on Saturday.
French Prime Minister Elisabeth Borne said France, Gabon's former colonial ruler, was following the situation closely.
If successful, the coup would be the eighth in West and Central Africa since 2020. The latest one, in Niger, was in July. Military officers have also seized power in Mali, Guinea, Burkina Faso and Chad.
Niger and other Sahel countries have been battling terrorist insurgencies that have eroded faith in democratic governments.
Gabon, which lies further south on the Atlantic coast, is not facing the same challenges, but a coup would suggest a further sign of democratic backsliding in a volatile region.
Bongo's family has ruled the oil producing but poor nation for 56 years. His detractors say he has done little to channel its oil and other wealth toward the population of some 2.3 million people, a third of whom live in poverty.
"Today the country is undergoing a severe institutional, political, economic, and social crisis," the officers said in a statement, saying the Aug. 26 election lacked transparency and credibility.
"In the name of the Gabonese people ... we have decided to defend the peace by putting an end to the current regime," they said.
One officer read the joint statement, surrounded by a dozen or so others in military fatigues and berets.
The servicemen introduced themselves as members of The Committee of Transition and the Restoration of Institutions.
The state institutions they declared dissolved included the government, the senate, the national assembly, the constitutional court and the election body.
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