Christiana Thorpe, head of the electoral commission, said Saturday's vote finished on time and without incident at most polling centres, despite rain and long lines.
Results will be released progressively, with final tallies within 12 days of voting.
Thorpe said she expected the first provisional results on Monday.
If none of the presidential hopefuls gets at least 55 per cent of the ballots cast, a second round of voting will take place.
Ahmad Tejan Kabbah, who has led the country as president for two five-year terms, was not eligible to stand for re-election.
Commonwealth observers said the people of Sierra Leone had been given the opportunity to "express their will freely and in accordance with internationally accepted standards".
However, there were some disturbances.
Late on Saturday, police had used tear gas to disperse crowds of youths setting up makeshift roadblocks the east of the capital city of Freetown.
It was unclear what sparked the incident or if it was related to the polling.
Thorpe also noted "a small number of violent incidents" in western Freetown, and said polling was delayed in one southern town because of lost ballot boxes.
Koroma told Al Jazeera the election was a "very good day for Sierra Leone".
He said he was confident of victory even if there was a further run-off vote to decide the presidency.
"I hope the people of Sierra Leon 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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The APC leader said: "We have also made substantial inroads. We have been welcomed in areas that are not normally our strongholds. I am confident of victory - even if there is a run-off I am going to win".
About 2.6 million voters were eligible to pick a new president and representatives, six years after the end of the Sierra Leone's civil war.
In all, seven political parties fielded candidates in a country ranked the second poorest on earth, despite its huge mineral resources including diamonds.
Legislators are elected by a simple majority, and 566 candidates stood for the 112 seats in the single-chamber parliament.
The elections are only the second since the country emerged from one of the most brutal wars in modern history, and the first poll Sierra Leone has organised after 17,500 UN peacekeepers pulled out of the country in December 2005.
A civil war, fuelled by blood diamonds, ravaged Sierra Leone between 1991 and 2001, claiming 120,000 lives while hundreds of thousands of survivors suffered horrors at the hands of fighters.