Opposition candidate and former military junta leader Julius Maada Bio was sworn in as Sierra Leone's new president late on Wednesday, just hours after the elections commission announced his victory in a tight run-off poll.

He now faces the difficult task of rebuilding the impoverished West African nation's economy that was dragged down by the world's deadliest Ebola epidemic and a global slump in commodity prices.

Representing the Sierra Leone People's Party (SLPP), Maada Bio won 51.81 percent of votes cast in the March 31 poll, according to results announced by the National Electoral Commission (NEC) on Wednesday.

He defeated former foreign affairs minister and ruling All People's Congress (APC) candidate Samura Kamara, who had held a slight lead based on partial results earlier in the day but in the end garnered 48.19 percent.

Dressed in traditional white robes, Maada Bio was sworn in just before midnight at a hotel in the capital Freetown, raising in the air the Bible upon which he swore the oath of office to the cheers of supporters.

"This is the dawn of a new era. The people of this great nation have voted to take a new direction," he said in a speech following the short ceremony in which he made an appeal for national unity.

"We have only one country, Sierra Leone, and we are all one people."

"EVERYONE IN, NO ONE OUT"

Maada Bio, who briefly ruled Sierra Leone as head of a military junta in 1996, replaces outgoing President Ernest Bai Koroma, who could not seek re-election due to term limits.

The largely peaceful election process has come as a relief for the country of 7 million people, who in the 1990s endured a brutal civil war fuelled by the diamond trade and notorious for its drug-addled child soldiers and punitive amputations.

SLPP supporters packed into the NEC headquarters on Wednesday, and following the announcement of the election results party officials urged the Maada Bio's backers to remain calm.

"Celebrate responsibly. Do not disturb your neighbour. Victory for all men, not victory for some. Everyone in, no one out," the party's campaign manager Ali Kabba said.

Opposition supporters, confident of victory, sang and danced in the streets of Freetown on Wednesday evening hours before Maada Bio was officially declared the winner.

"I feel happy about the results. I am here because my president Julius Maada Bio has won the election in this country," said Adolfus Kargbo, among a group of SLPP supporters chanting Maada Bio's name.