NAIROBI — A constitutional court in Congo issued a middle-of-the-night decision early Sunday affirming opposition candidate Felix Tshisekedi’s victory in a contentious presidential election that featured widespread irregularities, disenfranchisement and evidence of manipulation.

In an indication of how little the decision does to resolve Congo’s political crisis, another opposition candidate, Martin Fayulu, immediately declared himself president and said the court’s decision was “nothing more than a constitutional coup” while calling for “peaceful protests.”

The court rejected two petitions from Fayulu and another candidate claiming that the election commission had released fraudulent results and that it had illegally disqualified more than a million voters from casting ballots in regions stricken by the Ebola virus and ethnic conflict. According to the official results, Fayulu lost to Tshisekedi by fewer votes than those who were disenfranchised by that move, which affected areas likely to support Fayulu.

Tshisekedi, 56, is set to be inaugurated as Congo’s president on Tuesday. He would take over from Joseph Kabila, who has been president for 18 years since he inherited the position from his father, Laurent Kabila, who was assassinated in 2001. Tshisekedi is a political novice who has spent most of his life in the shadow of his father, Etienne Tshisekedi, the founder of Congo’s Union for Democratic and Social Progress, who died in early 2017.

Congratulatory notes poured in for Tshisekedi from African leaders including South Africa’s Cyril Ramaphosa and Kenya’s Uhuru Kenyatta. The African Union, which had expressed “serious doubts” about the veracity of the vote and planned to dispatch a delegation of leaders on Monday, canceled its mission.

Regional powers have feared that unrest caused by a disputed election could spill across Congo’s borders, as violence and displacement have in the past. At least 34 people have been killed since provisional results were released on Jan. 10, the United Nations said.

The endorsement of the court’s decision by regional power brokers all but ensures Tshisekedi’s victory, and so far it appears Fayulu’s supporters have not taken to Congo’s streets in large numbers. Tshisekedi’s inauguration is certain to be a festive affair, but if he is not able to prove his independence from Kabila, he may begin to feel backlash from within his party.

According to official results, Kabila’s party won a resounding majority of parliamentary seats, raising the possibility that Tshisekedi will be hamstrung in passing major reforms or holding Kabila and his advisers to account for alleged corruption stemming from Congo’s vast and lucrative mining sector.

Tshisekedi’s and Kabila’s parties dismissed leaked data from the election commission and the largest election observer mission, called Cenco, that showed Fayulu winning the Dec. 30 election with around 60 percent of total votes.

“It is Congo that won,” said Tshisekedi, speaking to his supporters after the court decision. “It is not the victory of one camp against another. I am engaged in a campaign to reconcile all Congolese . . . The Congo that we are going to form will not be a Congo of division, hatred or tribalism. It will be a reconciled Congo, a strong Congo that will be focused on development, peace and security.”

Fayulu told The Washington Post in a message that it is an “open secret” that Tshisekedi and Kabila have made a “backdoor arrangement” to share power, an allegation they fiercely deny. Tshisekedi’s silence while evidence mounted of irregularities in the election made many suspicious. His running mate, Vital Kamerhe, was once Kabila’s campaign manager. In an interview with The Post in December, Kabila did not rule out running for president in 2023.

The logic of a power-sharing deal rests on the assumption that Tshisekedi would be less threatening to Kabila as president than Fayulu, a former Exxon executive who has spoken forcefully about cleaning up corruption.

Congo is sub-Saharan Africa’s largest country by area and borders nine others. Its people speak four official indigenous languages. Native speakers of three out of four of them have served as heads of state, leaving out the region from which Tshisekedi hails.

Even amid doubts of the legality of Tshisekedi’s win, the fact that Kabila would no longer be president offered solace to many Congolese. Kabila’s approval rating had dropped below 20 percent in recent polls.

Just hours before the court made its decision known, the Congolese government instructed telecom companies to reestablish Internet connections and text messaging services, which had been blacked out for the past 20 days.