He was an ally and protege of the president's father, Omar Bongo, who ruled Gabon for 42 years until his death in 2009.
Gabon's main opposition parties chose former foreign minister Jean Ping as its candidate in an election on Aug. 27 against President Ali Bongo, who is standing for a second term in one of Africa's leading oil-producing nations.
Ping, aged 74, is considered one of Africa's foremost diplomats. His career has included a spell as chairman of the African Union commission and as president of the U.N. General Assembly.
He was an ally and protege of the president's father, Omar Bongo, who ruled Gabon for 42 years until his death in 2009. Ping fell out with Ali Bongo, who won a disputed election in 2009, and resigned from the ruling party in 2014.
"We think that this event will constitute the real departure point for change in this country and the start of a new Gabon," said Zacharie Myboto, who presided over the signing ceremony at which Ping was chosen over several other possible candidates.
Gabon's one-round electoral system is seen as favouring Bongo, though opposition unity could help overcome the president's institutional advantages accrued over the decades his family has held power.