Omar Bongo's reign among living heads of government - 37 years - is second only to Cuba's Fidel Castro, who has led the communist country for nearly 46 years.
In Africa, only Togo's late President Gnassingbe Eyadema served longer than Bongo - a 38-year run at the helm that ended in February after he died of a heart attack.
Most in Gabon believe Bongo, bolstered by a fractured opposition, personal wealth and decades of political patronage, is sure to win.
On a continent mired in war and beset by instability and conflict, Bongo has won praise for keeping Gabon peaceful.
During his campaign, he has made a point of saying he is the only man who can guarantee it remains that way.
Bongo has also been buoyed by Gabon's oil-wealth and timber cut from lush rain forests that have allowed the country to prosper.
Gabon produces about 290,000 barrels a day and boasts sub-Saharan Africa's third largest oil reserves, around 2.5 billion barrels according to US government figures.
Per capita income, according to the United Nations, is around $6,000 annually - roughly five to six times that of most African nations. Still, about half the population lives below the poverty line.
Most Gabonese believe Bongo will
win the presidential elections
Bongo has held a tight grip on power in this former French colony since 2 December 1967, when his predecessor Leon M'Ba - the country's only other head of state since independence from France in 1960 - died.
Bongo, then vice president, succeeded M'Ba and set up a one-party state. His stranglehold on politics was relaxed in the 1990s after pro-democracy protests swept across Africa after the end of the Cold War.
Before that, elections had been held, but Bongo was usually the only candidate on the ballot.
This time, Bongo is running against four other candidates.
Chief among them is opposition leader Pierre Mamboundou, a former postal worker who came in a distant second to Bongo in a 1998 vote the opposition said was rigged.
Also running are independent candidates Zacharie Myboto and Augustin Moussavou King, and little known former civil servant Christian Serge Maroga.
Zacharie Myboto is one of five
candidates running for president
Bongo's decades of experience have allowed him to easily dominate politics. In a televised debate ahead of Sunday's vote, Bongo declared simply: "I've made a name for myself, and people know me."
In the capital, Libreville, a city settled by rescued West African slaves in the 1840s, most believe Bongo cannot lose.
Marguerite Ndong, a 41-year-old health worker who was only three when Bongo came to power, said she was sure he would be elected. "Bongo has experience. We will elect him because we are afraid of the unknown."
On Friday, an initial round of voting began with about 15,000 security forces casting ballots ahead of the weekend poll so they would be free to carry out their duties on election day.
About 560,000 of Gabon's 1.4 million people are registered to vote on Sunday.
Security forces cast their ballots
ahead of the weekend poll
Polls open from 7am (0600GMT) to 6pm (1700GMT).
Bongo is likely stay at the pinnacle of Gabonese politics for the foreseeable future.
In 2003, parliament passed an amendment to the constitution ensuring there is no limit on the number of times a president can stand for office.