Meanwhile, Koroma expressed concern about the situation in Kailahun, a stronghold of the ruling party, where he said his workers had not been able to access some polling stations due to intimidation by their political rivals.
"Our local agents were harassed and it got to a point where they feared for their lives," he said.
Dozens of people were wounded in violent inter-party clashes ahead of Saturday's vote and security was high across the country on polling day.
"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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Election monitors said the second round of voting was more orderly than during an inconclusive first ballot held almost one month ago.
Electoral officials began gathering voting materials after polls closed with final results to be released within 12 days.
The race is expected to be a close one after Koroma led the first round August 11, winning 44 per cent of the vote, compared with Berewa's 38 per cent.
Neither candidate won the majority needed to win in the first round. but in the run-off, only a simple majority is required for victory.
Trailing in third place last time was Charles Francis Margai, a former cabinet minister, with 14 per cent.
Margai's party is now backing Koroma, leading many to believe he has a good chance of returning his party to power for the first time since it was ousted in a 1992 coup.
Brima Acha Kamara, a Freetown police chief, confirmed "pockets of disruption" in the capital, adding that patrols had been reinforced in potential trouble areas.
The military was also on hand to help police patrol the country after violence marred last week's campaigning.
On Friday, Ban Ki-moon, the UN secretary-general, appealed for calm, saying he was "deeply concerned about the recent incidents of harassment, intimidation and violence involving supporters of the two main Sierra Leone political parties"
Minutes before the end of voting on Saturday, a group of a dozen unidentified men tried to storm a polling station in Freetown's Lumley area after a stamp used to validate ballot papers was reported missing.
When scores of armed and anti-riot police were dispatched to the station, people began to throw stones, smashing the windscreen of at least one parked car.
Voting for change
Many in Sierra Leone, the world's second poorest nation, said they hoped the elections marked a change in the country's fortunes.
Phillip Alieu, a clergyman, said he was "voting for nothing but change".
"I don't think there are people anywhere else suffering as much as here in Sierra Leone and the only way to change that is to vote for a new government."
Tamba Sillah, whose right arm was hacked off by rebels during the decade-long civil war, said: "The past government did not do anything for us amputees. It was all promises, promises and promises."
Political tensions increased two weeks ago after results of the first round showed Berewa, the ruling party candidate, trailing second to Koroma, the former ruling All People's Congress candidate.
About 2.6 million voters, roughly half of the population, are registered to vote in Sierra Leone's second elections since the end of its civil war and its first since about 17,500 UN peacekeepers were pulled out in 2005.
Early signs, though, pointed to a lower turn-out than in the first round.
The first substantial results are expected on Monday.