Opposition leader Felix Tshisekedi will be sworn in on Thursday as Democratic Republic of Congo's next president, sources in Kinshasa said, as outgoing President Joseph Kabila called for national unity.In his first speech since the December 30 presidential election, Kabila said he was "calling for a grand coalition of all the progressive forces"."A coalition against the predatory forces that have come together and will always try to join forces to monopolise our natural resources," Kabila said on state channel RTNC.The inauguration will be held at the Palace of the Nation, the seat of the presidency, starting at noon (1100 GMT), aides to Tshisekedi and Kabila said on Wednesday, ending uncertainty about when the ceremony would take place.It will be the first peaceful transition of power in the history of the mineral-rich Democratic Republic of Congo, which gained independence from Belgium in 1960.Tshisekedi, 55, will be taking the helm from Kabila who at only 47 has ruled the vast country for 18 years, succeeding his father Laurent-Desire, who was assassinated in 2001.Tshisekedi was declared winner of the election with 38.5% of the vote, while his opposition rival Martin Fayulu was credited with coming a close second with 34.8%.But Fayulu says he was the true winner, with 61% of the ballot, against 18% each for Tshisekedi and Emmanuel Ramazani Shadary, Kabila's preferred candidate.Fayulu has angrily characterised the official result as an "electoral coup" cooked up by Kabila and Tshisekedi.But he lost a challenge at the Constitutional Court, and foreign support for his position has waned, mainly to avoid the bloody confrontations that have marred this vast country's history.Kabila congratulated Tshisekedi in his speech on Wednesday, saying he will hand over power to him "without regret or remorse"."He must be able to count on me whenever he wants and that the interest of the country requires." US hails peaceful transition The United States on Wednesday recognised Tshisekedi as the next president and hailed Kabila for enabling a historic peaceful transfer of power. It followed signals from the African Union and European Union that they too are ready to work with Tshisekedi, showing no appetite for prolonging the uncertainties in this violence-prone nation. "We are committed to working with the new DRC government. We encourage the government to include a broad representation of DRC's political stakeholders and to address reports of electoral irregularities," US State Department spokesperson Robert Palladino said in a statement.While France's Foreign Minister Jean-Yves Le Drian said on Thursday that "doubts" remain about whether Tshisekedi won the election, he stressed that the priority was to preserve peace and calm in the country. DRC, which covers an area akin to that of western Europe, lived through two regional wars in 1996-97 and 1998-2003, and its last two presidential elections in 2006 and 2011 were marred by deadly clashes.Tshisekedi is the son of the late veteran opposition leader Etienne Tshisekedi, who died in February 2017, aged 84.One of his first tasks in office will be to choose a prime minister, likely one of the pro-Kabila lawmakers who dominate the 500-seat National Assembly.Parliamentary elections were also held on December 30 alongside presidential and provincial polls.The Joint Front for Congo (FCC), which supports Kabila, controls 337 seats in the assembly while Fayulu's Lamuka coalition holds 102.The Heading for Change (Cach) coalition, which backs Tshisekedi, holds just 46, according to a provisional UN tally.The presidential inauguration had initially been scheduled for Tuesday but was postponed in order to give more time for preparations, notably invitations to foreign VIPs.Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.