Ex-rebel Democratic Republic of the Congo leader Jean-Pierre Bemba on Thursday officially launched his bid for the country's presidency, putting forward his candidacy for the long-delayed election in December.Another rival of long-serving President Joseph Kabila, opposition leader Moise Katumbi, was however barred from entering the country after saying he planned to return on Friday to contest the election.Bemba filed his papers at the electoral commission offices in the capital Kinshasa, throwing down the gauntlet to Kabila, who has ruled for 17 years.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook."I can confirm that I indeed had a voting card and I filed all the papers," Bemba said outside the electoral commission headquarters."Now it is up to the CENI (the national election board) to handle all the applications. For me, I have submitted the entire file, it is in order," he added.Bemba was accompanied by his wife and members of the Movement for the Liberation of the Congo (MLC), a former rebel group turned political party that Bemba founded.The 55-year-old, also a former DRC vice president, only arrived back in the country on Wednesday after 11 years abroad - a decade of it behind bars.He was greeted by large crowds of cheering supporters, many of them chanting anti-Kabila slogans.The International Criminal Court (ICC) in the Hague acquitted Bemba of war-crimes charges in June.UncertaintyAnalysts say his return has introduced even more uncertainty into an already volatile election process.Kabila was scheduled to stand down at the end of 2016 after his second elected term, technically the last permitted under the constitution.But he has stayed in office, invoking a constitutional clause enabling him to remain in power until a successor is elected.His time in charge has been widely criticised for rights abuses, corruption and incompetence.Bemba lost presidential elections to Kabila in 2006 and was later accused of treason when his bodyguards clashed with the army in Kinshasa.In 2007, he fled to Belgium, where he had spent part of his youth.He was then arrested in Europe on a warrant by the ICC for war crimes committed by his private army in the neighbouring Central African Republic from 2002-3, when its then-president Ange-Felix Patasse sought his help to repel a coup attempt.He was sentenced in The Hague in 2016 to 18 years before the conviction was overturned in June on appeal.The ICC declared Bemba could not be held responsible for crimes committed by his troops.The court is due to issue a ruling in a separate case, in which Bemba was sentenced to jail and fined 300 000 euros in 2017 for bribing witnesses during his main trial.But legal experts expect him to be released definitively if time spent behind bars is taken into account.'Security risks'Opposition leader Katumbi, who has been living in self-imposed exile in Belgium since May 2016 after falling out with Kabila, had planned to return on Friday to his former stronghold of Lubumbashi but the city's mayor said he would be refused entry.If he does return, however, he faces possible arrest after being handed a three-year jail term in absentia in June 2016 in a case of alleged property fraud, charges he denies.A leader of Katumbi's party, Ensemble, said there was concern about "security risks" in the light of "events" that occurred when Bemba returned to Kinshasa."There are risks of breakdowns which could put civilians and political figures such as Katumbi in danger," said Delly Sesanga, Ensemble's secretary general.Aides to Katumbi had met earlier on Thursday with United Nations officials to discuss his return, sources said.The UN peacekeeping mission to the DRC, MONUSCO, confirmed a meeting had taken place, in the context of its "good offices" in the country.But MONUSCO spokesperson Florence Marchal added that its "mission is not to ensure the protection of (individual) personalities".The DRC has never known a peaceful transition of power since it gained independence from Belgium in 1960 - and some experts fear the current crisis may spiral into bloodshed.Two wars unfolded from 1996-97 and from 1998-2003 that sucked in other countries in central and southern Africa.Smaller, but still bloody, conflicts dog the centre and east of the vast country today.