The election for a new president is seen as crucial to ending years of sectarian violence that has pitted Muslims against Christians. Two former prime ministers are battling it out for the top job.
Around 10,000 security forces - including UN peacekeepers - were deployed in the capital Bangui and several other provinces ahead of Sunday's presidential run-off.
Security was stepped up amid concerns that tensions could re-ignite rivalries between the militias responsible for more than two years of violence.
Two former prime ministers, Faustin-Archange Touadera and Anicet-Georges Dologuele, are contesting the run-off after leading the ballot in December.
Dologuele received about 24 percent of the votes in the first round, while Touadera also gained strong grassroots support amid hopes the country can restore democratic rule.
Officials will also attempt to re-run a first round of legislative polls on Sunday, which were annulled because of irregularities.
Unity and stability promised
Both candidates have campaigned on messages of peace and unity in an attempt to avoid a repeat of the worst crisis in the country's history. In early 2013, mainly Muslims Seleka fighters toppled President Francois Bozize.
Christian militias then responded by attacking the Muslim minority community, which led to thousands of deaths and left nearly a million people displaced.
Tens of thousands of voters will be casting ballots from refugee camps in neighboring Cameroon and Chad.
"I really hope the elections take place so that the people who have had to flee their homes and are suffering in the camps can go back to their homes, have peace and live as they should," said Bangui resident Lina Mbelery.
A turnout of nearly 80 percent for the first round of voting in December was seen as a popular rejection of the violence, which has allowed Muslim rebels to control the northeast while Christian militias roam the southwest.
Long road ahead
The Central African Republic has been led by a transitional government for the past two years. Whoever wins the presidency will face the daunting tasks of extending state authority outside the capital, rebuilding the army and kickstarting a failing economy.
"(Elections) are not going to solve the deep, systemic problems that put this country into conflict," said Lewis Mudge, Africa researcher for Human Rights Watch. "It's cheaper to buy a grenade in Bangui than it is to buy a can of Coke. That's how bad it is here."
mm/jm (AP, Reuters)