Sierra Leone's opposition candidate, Julius Maada Bio, has rushed to take his presidential oath in a hotel after winning the run-off election.
Mr Maada Bio is a former military ruler who briefly ruled the country in 1996.
He narrowly beat ruling party candidate Samura Kamara, who has alleged irregularities and says he will challenge the outcome in court.
Mr Maada Bio was sworn in on Wednesday, less than two hours after being declared the winner of Saturday's vote.
"[This is] the dawn of a new era," he said. "The people of this great nation have voted to take a new direction."
In a televised address, Mr Kamara said: "We dispute the results and we will take legal action to correct them." He also urged his supporters to stay calm.
Mr Maada Bio, leader of the Sierra Leone People's Party (SLPP), won just under 52% of the vote.
How did Maada Bio come to power before?
Mr Maada Bio, now 53, was part of a group of soldiers who overthrew the government in 1992 when he was in his late 20s.
Then, in January 1996, he staged a palace coup, arguing that his boss, Capt Valentine Strasser, wanted to renege on the promised handover to an elected civilian government.
His supporters point to that to call him the "father of democracy".
But his critics cite human rights violations witnessed while he was in power, for which he has taken "collective responsibility".
During his rule, which lasted just over two months, he appointed Mr Kamara as his finance minister.