The nation, dogged by coups, violence and misrule since winning independence from France, could take a step towards rebirth if the polls and their aftermath go smoothly.
"It's crucial that people vote today," said Paterne, a voter in his 40s, as he queued at a polling station in the capital Bangui. "For the first time, we have a true opportunity to turn our backs on war."
The two men in the close presidential race are both former prime ministers who have campaigned on promises to restore security and boost the economy in the mineral-rich but dirt-poor country.
Anicet Georges Dologuele, a 58-year-old former central banker known as "Mr Clean" for his efforts to bring transparency to murky public finances, won the first round on December 30, taking 23.78 per cent of the vote.
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He faced Faustin Archange Touadera, a former maths professor, in the run-off. Also 58, Touadera was standing as an independent and surprised everyone by coming second in the first round with 19.4 per cent.
Dologuele wished voters a happy Valentine's Day as he cast his ballot in Bangui.
"Valentine's is a celebration of love, and I'd like Central Africans to see voting today as an act of love for their country."
He spoke of the "joy of being able to vote in the second round and in doing so, to participate in the transition and the start of a new era for the Central African Republic".
Touadera, speaking to voters near the working-class neighbourhood of Boy Rabe, pitched himself as the people's candidate.
Central Africans also voted in a re-run of the last legislative election, also held on December 30, that was later annulled over numerous irregularities.
A total of 1,800 candidates were competing for 105 seats in the National Assembly.