The electoral commission announced overnight that Tshisekedi, 55, had won the December 30 vote
Congo's Catholic Church, one of the country's most respected institutions, challenged Thursday's official results from a chaotic presidential election, suggesting its tally did not give victory to opposition leader Felix Tshisekedi.
Though bishops stopped short of naming a winner, the announcement sets up a potential showdown with President Joseph Kabila's government over a poll many fear could provoke more violence in the vast and volatile nation of 80 million people.
The electoral commission (CENI) announced overnight that Tshisekedi, 55, had won the December 30 vote, edging out another challenger, businessman Martin Fayulu.
"The results from the presidential election as published by CENI do not correspond to the data collected by our observation mission from polling stations and vote counts," the National Episcopal Conference of Congo observers said in a statement.
Both France and Belgium expressed doubts. And three diplomats briefed on the findings of the 40,000-strong Catholic observer mission told Reuters they showed Fayulu winning.
He said Kabila had engineered an "electoral coup" to deny him the presidency. Nevertheless, supporters of Congo's president-elect were celebrating their unlikely win.
Though most of the nation seemed calm on Thursday, at least two people were killed in violence at a western town.
Anger over the results, and the Fayulu camp's suspicions Tshisekedi won by cutting a power-sharing deal with Kabila, have cast a cloud over what is meant to be Congo's first democratic transfer of power in 59 years of independence.
Tshisekedi's camp has acknowledged contact with Kabila's representatives since the election but said they were aimed at ensuring a peaceful transition and denied a deal.
In contrast to previous polls, election officials did not provide a regional breakdown of the results.
In the town of Kikwit, 500km (310 miles) from the capital Kinshasa, security forces opened fire after crowds attacked symbols of government. At least two people died in the melee, a local journalist and a UN source said.
There were celebrations in parts of Kinshasa and the south of the country, where Tshisekedi has broad support. Towns in Katanga, the eastern mining heartland, were calm.
But protests were reported in the central town of Kisangani, and Fayulu supporters vented their frustrations.
"We will never accept this nomination. It's not a victory for Felix. CENI has appointed him," said Georges Bingi, a member of Fayulu's party in the eastern city of Goma.
Fayulu may appeal to Congo's constitutional court.
By contrast, Kabila's hand-picked candidate, Emmanuel Ramazani Shadary, who finished a distant third, conceded.
"Of course we are not happy as our candidate lost, but the Congolese people have chosen," Shadary spokesman Barnabe Kikaya Bin Karubi told Reuters.
Tshisekedi's win raises questions over the future of Kabila, who has governed since his father's assassination in 2001 and overstayed the official end of his mandate by two years.
Kabila says he wants to remain involved in politics and may run again in 2023, when he will no longer be term-limited.
Tshisekedi inherited the leadership of the UDPS party when his father, Etienne, died in 2017. But he lacks the experience, political clout and firebrand reputation his father earned campaigning for democracy under three successive presidents.
No details of any deal have emerged. But in the run-up to the results, Tshisekedi said Kabila had nothing to fear should he come to power, comments analysts interpreted as efforts to reassure the president and his supporters that interests accumulated over two decades in power were not at risk.
Fayulu, however, is backed by ex-rebel Jean-Pierre Bemba and former governor Moise Katumbi, two of Kabila's fiercest rivals.
Any escalation in the Fayulu camp's rhetoric or actions risks igniting Congo's cycle of unrest, particularly in the eastern borderlands where he enjoyed strong support and dozens of militia groups are active.
It is unclear whether voting will ever be held in parts of eastern Congo where the election was delayed - disenfranchising more than 1.2 million people - due to concerns about Ebola and violence. Tshisekedi's margin over Fayulu was less than 700,000 votes, according to the official results.
Members of Tshisekedi's UDPS party called the election a historic triumph in a struggle for democracy.
The inauguration had been scheduled for January 18.
"Today marks the liberation of the people," said Simphora Biduaya, a candidate for provincial assembly in the east.
But French Foreign Minister Jean-Yves Le Drian demanded clarity on results "which are the opposite to what we expected." And former colonial power Belgium said it would use its temporary seat on the U.N. Security Council to also seek clarification.
If Tshisekedi's victory is confirmed, he will become the first leader to take power at the ballot box since Prime Minister Patrice Lumumba. He was toppled in a coup less than three months after independence in 1960 and killed four months later.