Gen Brice Oligui Nguema has been named the head of a claimed transitional government in Gabon after military officers led an armed takeover of the country early on Wednesday morning.
A group of senior Gabonese military officers appeared on TV to announce the coup, after the state election body announced President Ali Bongo had won a third term.
The President has been placed under house arrest and his son has also been detained.
The military officers said they were closing Gabon's borders until further notice and dissolving state institutions.
UN Secretary General Antonio Guterres “firmly condemns the ongoing coup attempt as a means to resolve the post-electoral crisis”, he said in a statement provided by spokesman Stephane Dujarric.
Mr Guterres expressed “deep concern over the announcement of the election results amidst reports of serious infringements of fundamental freedoms” and said the UN “stands by the people of Gabon”.
Gen Nguema replaced Mr Bongo's step-brother in October 2019 as head of Gabon's Republican Guard, the elite force in charge of protecting the president, his family and other high-profile figures.
He is from Gabon's south-easternmost province of Haut-Ogooue, on the border with the Republic of Congo, the same part of the country as the ousted president.
The general won his spurs as an aide-de-camp to the ousted leader's father, Omar Bongo, who ruled Gabon for almost 42 years until his death in 2009.
The announcement on the Gabon 24 channel came shortly after the Gabonese Elections Centre declared Mr Bongo, who has been in power for 14 years, had won a third term in elections on Saturday.
“The president of the transition insists on the need to maintain calm and serenity in our beautiful country,” Lt Col Ulrich Manfoumbi said on state TV.
“At the dawn of a new era, we will guarantee the peace, stability and dignity of our beloved Gabon.
Another of the officers said: “We have decided to defend peace by putting an end to the current regime.”
He said he was speaking on behalf of the “Committee for the Transition and Restoration of Institutions”.
“To this end, the general elections of August 26, 2023, and the truncated results are cancelled,” he said.
“All the institutions of the republic are dissolved – the government, the Senate, the National Assembly and the Constitutional Court.”
Gabon also resumed the broadcasts of several French media outlets that had been suspended, the televised statement said.
The “temporary broadcast ban” affected France 24, RFI and TV5 Monde, which were “accused of a lack of objectivity and balance … in connection with the current general elections”.
After the announcement, hundreds of people celebrated the end of Mr Bongo's reign in the centre of Gabon's capital, Libreville, where they sang the national anthem with soldiers.
Shopkeeper Viviane Mbou offered the soldiers juice, which they declined.
“Long live our army,” said Jordy Dikaba, a young man walking with his friends on a street lined with police in body armour.
Gabon’s government announced a nationwide curfew and cut off internet access on Saturday evening, as voting in major national elections for new local leaders, national legislators and the next president was wrapping up.
Nigerian President Bola Tinubu, the chairman of West African bloc Ecowas, is working closely with other African heads of state on how to respond to the situation in Gabon.
Mr Tinubu is watching developments in Gabon and the “autocratic contagion” spreading across the continent “with deep concern”, a representative said, referring to a recent coup in Niger.
The African Union has also called on Gabon's security forces to guarantee the safety of Mr Bongo.
The military takeover in Gabon came only weeks after members of the presidential guard in Niger seized power and established a junta.
EU foreign policy chief Josep Borrell, speaking at a meeting of the bloc's defence ministers in Spain, said the coup “increases instability in the whole region”.
“The whole area, starting with Central African Republic, then Mali, then Burkina Faso, now Niger, maybe Gabon, it's in a very difficult situation and certainly the ministers … have to have a deep thought on what is going on there and how we can improve our policy in respect with these countries,” Mr Borrell said.
Olivier Vera, spokesman for the French government, said: “We condemn the military coup and recall our commitment to free and transparent elections.”
Mr Bongo, 64, was elected president in 2009 after the death of his father, Omar Bongo Ondimba, who had been in power since 1967.
He received 64.27 per cent of votes cast in the single-round ballot on Saturday, beating his main rival Albert Ondo Ossa, who won 30.77 per cent of the vote, and 12 other candidates, the head of the electoral authority, Michel Stephane Bonda, announced on state TV.
Voter turnout was 56.65 per cent.
Every vote held in Gabon since the country’s return to a multiparty system in 1990 has ended in violence.
Four people were killed in clashes between government forces and protesters after the 2016 election, according to official figures. The opposition said the death toll was far higher.
In anticipation of post-electoral violence, many people in the capital went to visit family in other parts of the country or left Gabon altogether. Others stockpiled food or bolstered security in their homes.
