KINSHASA (Reuters) - Democratic Republic of Congo opposition leader Jean-Pierre Bemba filed his candidacy on Thursday for December’s presidential election, cementing an unlikely comeback after a decade in prison for war crimes.

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Bemba returned to Congo on Wednesday to a hero’s welcome from tens of thousands of his supporters who lined the streets of the capital Kinshasa and chanted slogans against President Joseph Kabila.

His war crimes convictions at the International Criminal Court (ICC) in The Hague were unexpectedly quashed on appeal in May, and he could now pose a stern challenge to Kabila or his preferred successor in the long-delayed vote.

“I announce to you that I will be your candidate in the 2018 presidential election,” Bemba wrote on Twitter after submitting his candidacy at the national electoral commission headquarters.

It is not clear, however, if the courts will approve Bemba’s candidacy. Kabila’s ruling coalition said last week he should be excluded from the race due to a witness tampering conviction that was not annulled by the ICC.

Moreover, Kabila, in power since 2001, has refused to commit publicly to stepping down even though his mandate ran out in 2016. This has raised fears he will defy constitutional term limits and spark a violent backlash by the population and militia groups active in the country’s east.

Bemba was a warlord with a militia in northern Congo during the country’s second war, which began in 1998, and he also served as a vice president to Kabila for three years until 2006.

Another opposition hopeful, millionaire businessman and former provincial governor Moise Katumbi, said he plans to return to Congo on Friday after two years in exile but risks arrest due to a conviction for real estate fraud in 2016.

Government spokesman Lambert Mende said Katumbi has no authority to land in Congo and that he would be arrested if he arrives by commercial flight.

“The Attorney General of the Republic issued a mandate to arrest the fugitive Moise Katumbi, ordering his immediate arrest once on Congolese soil,” he said.