The main opposition parties in Gabon have agreed to put forward a joint candidate for next week's presidential election to challenge incumbent Ali Bongo Ondimba, the head of the alliance announced Friday.

Francois Ndong Obiang, president of the Alternance 2023 opposition grouping, said that the main opposition figures, who had previously been looking at selecting their own separate candidates, had surprised everyone by picking a "consensus" candidate.

The man who will attempt to unseat Bongo is former education minister Albert Ondo Ossa.

President Bongo's family has ruled the oil-rich West African state for 55 years.

"You have before you the consensus candidate", declared 69-year-old Ondo Ossa, in front of the few militants gathered at the headquarters of the opposition Reagir party.

"I'm particularly moved and I'd like to thank all the party presidents", he added, calling for strong mobilisation with so little time left before the August 26 polls.

Launched in January, Alternance 2023 brings together six opposition groupings and has regularly held talks with the aim of establishing a joint candidacy, the only way, in its view, of avoiding splitting the vote to the benefit of the incumbent.

The Gabonese Election Centre had already validated 19 of the 27 candidacy applications received, five more than in 2016.

Far from being the favourite, Ondo Ossa had been up against leading rivals including Alexandre Barro Chambrier of the opposition Rally for the Fatherland and Modernity (RPM) party and the National Union's head Paulette Missambo as well as a former Bongo prime minister Raymond Ndong Sima.

But now the six candidates belonging to the opposition alliance have all undertaken to withdraw their candidacy in favour of the consensus candidate, Ndong Obiang assured.

The 64-year-old President Bongo, who took over from his father Omar Bongo Ondimba in 2009, officially announced in July that he would run again for president.

The president was narrowly re-elected in 2016, with just 5,500 more votes than rival Jean Ping, who claimed the election had been fixed.

Bongo suffered a stroke in 2018 and spent months on the sidelines recovering, leaving the opposition to question his fitness to run the nation.