Sierra Leone's new president, the opposition challenger Julius Maada Bio, on Thursday began work on the transition of power after being declared winner of a bitterly-contested election run-off.Bio won 51.81% of ballots in last month's election in the West African country, according to the official results released late on Wednesday.A former soldier who briefly led a military junta more than two decades ago, he began on Thursday his first presidential term as a civilian."There is a meeting between Mr Bio and former President [Ernest Bai] Koroma at President Lodge... to discuss the transition team," the spokesperson for Bio's SLPP party, Ali Kabbah, told AFP. Bio defeated Samura Kamara, presented as successor to the outgoing Koroma and candidate of the ruling All People's Congress (APC), which has been in power for a decade.He was sworn in and handed a symbolic baton of command by the country's top judge at a hotel on Wednesday evening. Kamara secured 48.19% of the vote, but swiftly challenged the outcome given by the National Election Commission (NEC), saying in a televised address: "We dispute the results and we will take legal action to correct them." The results, he added, "do not reflect the party's many concerns about massive ballot box stuffing, supernumerary votes, and other irregularities."The United States congratulated Bio on his victory and praised the NEC for the "orderly, well-managed" elections. A statement from the US embassy in Freetown said: "As a longstanding friend, the United States congratulates the people of Sierra Leone on their active and peaceful participation in the presidential elections."Supporters celebrate Hundreds of people wearing the green colour of the SLPP sang, danced and blew horns around a central square in Freetown, where one of the country's emblems, the "Cotton Tree", grows.Alhaji-a Bah, a 19-year-old former soldier, said: "We need changes.""Sierra Leone is an undisciplined country... There are no judiciary, no discipline. Irresponsible government workers. Another supporter, Abu Amadu, said he voted for Bio's manifesto pledges of free education and improved healthcare. The Koroma administration leaves power with a mixed record. Sierra Leone is of the world's poorest nations despite huge mineral and diamond deposits, and it is recovering only gradually from war and disease. Its economy remains fragile with investors slowly returning, although corruption is widespread in the former British colony.Political loyalties are often divided along ethnic lines and traumatic memories of the 1991-2002 civil war run deep. Ugly campaign The election campaign was characterised by ugly verbal exchanges and sporadic violence with Bio accusing the APC of using police intimidation against his party.Police reported a string of attacks on candidates and supporters on both sides since the first round on March 7 - which Bio narrowly won - after which Kamara declared that "the safety and security of Sierra Leone is in our hands".KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. Bio, a straight-talking retired brigadier, blasted the government's closeness to China, while Kamara presented himself as a continuity candidate.A total of 3.1 million people were registered to vote in the first presidential poll since a 2014-2016 Ebola outbreak that killed 4 000 people.Koroma's term was also marred by a mudslide that struck Freetown last year, killing hundreds of people.Bio was in a group of young soldiers behind a 1992 coup that would install their leader, Valentine Strasser, as the youngest head of state in the world, at age 25.He later took power but agreed to step aside in 1996 for an elected civilian leader, and subsequently apologised for his role in the junta.