Ali-Ben Bongo is favourite to succeed his father
One of the 23 presidential candidates in Gabon has begun a hunger strike to demand that the election be postponed.
Bruno Ben Moubamba, an independent candidate, is one of 11 presidential hopefuls to have complained of irregularities in the voting register.
They also say limiting the 30 August vote to a 15 day campaign is rushed.
The favourite to win is Ali Ben Bongo, son of the late president Omar Bongo, who is standing as candidate of the ruling Gabonese Democratic Party (PDG).
Omar Bongo ruled Gabon from 1967 until his death in June.
Mr Moubamba helped launch a much-publicised manifesto in December 2008, which severely criticised President Bongo, his daughter Pascaline, who was his chief of staff, and Ali, who was then defence minister.
"I am staying in the process, but I will not be on the campaign trail for now," Bruno Ben Moubamba told AFP new agency.
Mr Bongo is due to hold his first campaign rally on Sunday in the capital, Libreville, two days after the interim president dismissed him as defence minister.
Led Gabon for nearly 42 years
Oil money means Gabon officially one of richest countries in Africa
Maintained close ties to former colonial power France
He denied corruption charges in French courts
Allowed multi-party polls in 1993; opposition said they weren't fair
Analysts say Mr Bongo will be the strong favourite to succeed his father but his ambition has proved divisive.
His late father's Prime Minister, Jean Eyeghe Ndong, is among several former ministers running against him.
Rights groups have expressed fears of a dynastic rule in the oil-rich nation.
Omar Bongo amassed a vast fortune during his years in office - but most of the 1.4 million people in Gabon live in poverty.
He maintained close economic and political links with former colonial power France, but French prosecutors launched an investigation into Mr Bongo's wealth in the months before his death.
He was accused of embezzling oil revenues and bribery.