Ali Bongo Ondimba took power in 2009 following the death of his father, Omar Bongo, who ruled the oil-producing country of about 1.5 million people for more than 40 years.
In his campaign Bongo has touted his efforts to diversify Gabon's economy and accused the opposition of inciting violence.
The most formidable challenge among nine opponents comes from Jean Ping, a former chair of the African Union Commission who managed to get several other aspirants to rally around his candidacy.
The tense campaign has featured efforts to get Bongo's candidacy annulled based on claims he was born in Nigeria and therefore is ineligible to be president claims Bongo has dismissed as unfounded.
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Bongo's victory in 2009 sparked looting and clashes between protesters and security forces.
The office of UN Secretary-General Ban Ki-moon yesterday issued a statement calling for a peaceful vote, urging candidates "to exercise restraint, abstain from any acts of incitement or the use of inflammatory statements, and maintain a peaceful atmosphere before, during and after the election."
Polling stations were scheduled to open at 7 am, though several in the capital, Libreville, opened 30 or 40 minutes late because political party representatives failed to show up on time.
Voting is scheduled to end at 6 pm, with provisional results expected early next week.
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