Gabonese President Ali Bongo has campaigned for re-election in remote corners of the rainforested nation this month, seeking to dispel opposition claims he lacks the strength and track record to fulfil generous electoral promises in a third term.

The Central African state holds presidential, legislative and local elections on August 26. Of Bongo's 18 competitors, six have backed a joint candidate in an effort to marshal the opposition vote and end his family's 56-year grip on power.

The vote is a much-anticipated test of support for Bongo. Detractors say he has done too little to funnel Gabon's oil wealth towards the third of its 2.3-million population living in poverty and question his fitness to govern after a stroke in 2018.

On his wide-ranging campaign trail, the 64-year-old has tried to disprove this image. Wooing voters with promises of higher family subsidies and cuts to public school fees, Bongo has stepped out confidently on stage - a contrast to his rare and frail television appearances in the wake of his illness.

"We will win it, because I have a vision ... of what the future of Gabon should be. I heard you. I know where your priorities are. I know where you want us to go all out," he said at a rally on Monday.

His main challenger is Albert Ondo Ossa. The 69-year-old economics and management professor was picked by an alliance of six main opposition parties' as their joint candidate last Friday, barely a week before the vote.

His campaign has focused on the need for change and better economic opportunities - a potentially enticing offer in a country where one in three young people are unemployed and the vast majority of the population has only known Bongo rule.

"We have to manage the country differently. Our youth has the right to have something else, especially in this country of such immeasurable wealth," Ondo Ossa said when he won the joint nomination.

"Gabon is not the property of the Bongos."

'INTO THE UNKNOWN'

Tensions are running high ahead of the vote amid fears recent electoral system changes could sow doubt about the validity of the result and provoke unrest. Deadly clashes after Bongo's 2016 victory saw the parliament building gutted by fire.

Both of Bongo's election wins have been disputed by the opposition, which says he won fraudulently.

Ondo Ossa's firebrand reputation means he is likely to cry foul over any sign of electoral irregularity, said Modeste Abagha, political analyst at the National Center for Scientific and Technological Research in the capital Libreville.

"We're going into the unknown, as Albert Ondo Ossa is very cantankerous," Abagha said. "He won't be robbed of victory ... We are not heading towards a peaceful tomorrow if the ballot does not take place as it should."

The opposition has already voiced its concern over a new single ballot system, which mean voters have to pick a presidential candidate and lawmaker from the same party. It also opposes a recent constitutional change to abolish two rounds of voting for the president, claiming this favours Bongo.

Bongo's camp has positioned him as the firm favourite to win the race, although there has been no reliable polling.

His defeat would be a major upset, not only in terms of the end of his family's decades-long dynasty.

It could also derail Bongo's ambitious environmental agenda, which has protected Gabon's share of the Congo basin rainforest and helped the country remain one of the world's few net absorbers of climate-warming carbon dioxide.

The streets of Libreville have remained peaceful in the run-up to the vote with campaigning due to wrap up on Friday.

First-time voter Alban Mpiga said he would vote for Bongo: "If he had not had the illness that we all know about, I think that today we would be further ahead in terms of development."

Others were not convinced. "His father left us a catastrophic country. He too in 14 years has done nothing. That's why I will vote against him," said city council worker Gaetan Moussavou.