(AFP) - Leaders of the coup in Gabon on Wednesday named a general transitional president after seizing power following disputed elections in which President Ali Bongo Ondimba, whose family has ruled for 55 years, was declared the winner.

The claimed takeover sparked condemnation from the African Union and alarm from Nigeria over "contagious autocracy" in a continent where military forces have seized power in five other countries since 2020.

Bongo, 64, who took over from his father Omar in 2009, was placed under house arrest and one of his sons was arrested for treason, the coup leaders said.

TV images showed the head of the Republican Guard, General Brice Oligui Nguema, being carried triumphantly by hundreds of soldiers, to cries of "Oligui president".

The coup leaders later named Oligui Nguema "transitional president", according to a TV statement.

The coup began with a pre-dawn address in which a group of officers declared that "all the institutions of the republic" had been dissolved, the election results cancelled and the borders closed.

"The country is going through a serious institutional, political, economic and social crisis," according to a statement read on state TV.

It was read by an officer flanked by a dozen army colonels, members of the elite Republican Guard, regular soldiers and others.

The elections "did not meet the conditions for a transparent, credible and inclusive ballot so much hoped for by the people of Gabon", the statement said.

"Added to this is irresponsible and unpredictable governance, resulting in a continuing deterioration in social cohesion, with the risk of leading the country in chaos.

"We -- the Committee for the Transition and Restoration of Institutions (CTRI) on behalf of the people of Gabon and as guarantors of the institutions' protection -- have decided to defend peace by putting an end to the current regime," it said.

- Arrests -

Bongo's son and close adviser Noureddin Bongo Valentin, his chief of staff Ian Ghislain Ngoulou, his deputy, two other presidential advisers and the two top officials in the ruling Gabonese Democratic Party (PDG) "have been arrested", a military leader said.

They are accused of treason, embezzlement, corruption and falsifying the president's signature, among other allegations, he said.

A worried-looking Bongo, in a video from an unidentified location, appealed to "all friends that we have all over the world... to make noise" on his behalf.

"My son is somewhere, my wife is in another place and I'm at the residence and nothing is happening. I don't know what's going on. I'm calling you to make noise."

On the streets of the capital, and the economic hub Port-Gentil, groups of joyous people were seen celebrating.

In Libreville, around 100 people shouted "Bongo out!" and applauded police in anti-riot gear, an AFP staff member saw.

- Disputed election -

Bongo was first elected in 2009 following the death of his father Omar, who had ruled the country for 41 years, reputedly amassing a fortune.

The coup announcement came just moments after the national election authority declared Bongo had won a third term in Saturday's election with 64.27 percent of the vote.

Gabon's main opposition, led by university professor Albert Ondo Ossa, had accused Bongo of "fraud", and demanded that he hand over power "without bloodshed".

Authorities at the weekend imposed a curfew, which on Wednesday was extended until further notice, and shut down the internet nationwide. The internet was restored on Wednesday morning after the TV address.

Several French media outlets, whose output was suspended during the tumultuous period following the election, were permitted to resume, according to a statement from the coup leadership broadcast Wednesday.

Gabon's 2016 elections were marked by deadly violence after Bongo edged out rival Jean Ping by just 5,500 votes, according to the official tally.

In 2018, Bongo suffered a stroke that sidelined him for 10 months and fuelled accusations he was medically unfit to hold office.

- Family rule -

The central African country of 2.3 million people has been ruled by the Bongos for more than 55 of the 63 years since independence from France in 1960.

International reaction to the coup has been swift: the United Nations on Wednesday firmly condemned the coup, calling on the military to guarantee the safety of Bongo and his family.

The US State Department late Wednesday said it was "deeply concerned" and remained "strongly opposed to military seizures" while voicing concerns over the "lack of transparency and reports of irregularities surrounding the election".

Last week's election was held without international observers and Paris-based media rights campaigners Reporters Without Borders denounced the fact that foreign journalists had been largely restricted from covering the event.

The African Union said it "strongly condemns" the claimed takeover as a violation of its charter, while Nigerian President Bola Tinubu said he was in contact with African leaders over the "contagious autocracy we have seen spread across our continent".

Since 2020, there have been military takeovers in Mali, Guinea, Sudan, Burkina Faso and Niger.

Russia also said it was "deeply concerned", while China called for "all sides" in Gabon to guarantee Bongo's safety.

In France, where Bongo's loss would mark a blow to Paris' reach in Africa, the government said it "condemns the coup", reiterating its desire "to see the results of the election respected, once they are known".

Germany and Britain on Wednesday both condemned the takeover, with Berlin saying there were "legitimate" concerns over the elections.

© Agence France-Presse