The election is a key test of stability for the West African nation after more then a decade of civil war.
Political tensions rose two weeks ago after first round results showed Berewa trailing behind Koroma, leader of the opposition All People's Congress (APC), by six per cent.
The winner will succeed Ahmad Tejan Kabbah who will step down after two terms as president.
"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
Send us your views
Some 2.6 million were registered to vote in Saturday's polls, the second since Sierra Leone's brutal civil war ended six years ago.
The election is the first since some 17,500 UN peacekeepers withdrew in 2005.
Many said they voted for a new order in the country which, despite its huge diamond reserves and vast mineral wealth, remains the world's second-poorest nation.
The National Election Watch (NEW), the largest observer team representing civil society groups, said the polls were more orderly than the first round in August, although there were still reports of fraud.
"NEW is aware of some incidents around the country related to chaos in certain polling stations, ballot stuffing and violation of voting procedures," it said in a preliminary report.