During the first round in August, both candidates did not achieve the 55 per cent needed to win.
Koroma had won 44 per cent of the vote, compared to 38 per cent for Berewa.
"I hope the people of Sierra Leone can have this election without violence. All that is needed is one act of violence to spark another civil war."
Tom Dougherty, Atlanta, USA
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Trailing third was Charles Francis Margai, a lawyer and former minister, with 14 per cent.
A majority is needed by either party to win on Saturday's elections.
Kabbah was prevented by law from running for a third five-year term.
However, Margai's party broke away from the governing coalition and is backing Koroma.
The All People's Congress party, led by Koroma, had already won a majority in the 112-seat parliament, taking 59 seats, compared with the Sierra Leone People's [arty, led by Berewa, which had won 43 seats.
Both candidates have promised to crack down on corruption and unemployment, especially among local youths, who include ex-fighters and former child soldiers.