Careful statements by the worldwide community did not congratulate Tshisekedi but merely took note of the official results and urged peace and stability in a country with little of it. Observers appeared to be watching for how Fayulu's supporters reacted.
A chaotic vote in the vast and volatile nation of 80 million people has raised fears of renewed violence and at least two people were killed in clashes at one town in the west.
Vote tallies by Congo's Catholic Church had Favulu as the victor, according to two diplomats briefed on the findings.
Pierre Englebert of the African Arguments newsletter said analysis of survey data "suggests that the probability Tshisekedi could have scored 38 percent in a free election is less than 0.0000".
"This is not a bad result at all for the ruling party", said Stephanie Wolters, an analyst at the Institute for Security Studies, in South Africa.
Belgium said it would use its temporary seat on the U.N. Security Council to also seek clarification.
"We must have clarity on these results, which are the opposite of what we expected, " Le Drian told CNews.
"Outgoing President Joseph Kabila will be able to influence Tshisekedi, who now owes his ascendancy to power to Kabila's control of the electoral commission", Robert Besseling, a Johannesburg-based executive director at business risk consultancy Exx Africa, said in an emailed note.
Fayulu can appeal the results to Congo's constitutional court but has not yet indicated whether he will.
Spokeswoman Marie-France Idikayi tells The Associated Press that "we are waiting for the final deliberations of the electoral commission plenary session to end but the announcement room is prepared".
Rival opposition leader Martin Fayulu immediately disputed Tshisekedi's victory, calling it "an unacceptable electoral fraud".
The announcement of DRC's 30 December election results will likely be delayed to March or April (while four excluded localities vote), effectively extending the administration of incumbent President Joseph Kabila through this period.
North Korean diplomat in hiding after defecting, South Korean spy company says
The United States has been pressing for North Korea to get rid of its nuclear weapons before any easing of economic pressure.
"Mr Kabila is a Congolese citizen".
The electoral commission was meant to publish results last Sunday but said it needed more time.
Residents celebrate in Kinshasa, Congo, Thursday, Jan. 10, 2019, after learning that opposition presidential candidate Felix Tshisekedi had been declared the victor of the elections.
Kabila has ruled since 2001 in the troubled nation rich in the minerals key to smartphones around the world.
Election chief Corneille Nangaa declared Tshisekedi the victor with 38.57 percent of the 30 December vote, just ahead of Fayulu with 34.8 percent.
Felix Tshisekedi (right) and his running mate Vital Kamerhe campaign in Kinshasa, November 2018.
Congo's government cut internet service the day after the vote to prevent speculation on social media.
"This is the coronation of a lifetime", said the deputy secretary-general of Tshisekedi's party, Rubens Mikindo. "They have stolen the Congolese people's victory and the people will never accept that", said Fayulu, an oil tycoon whose platform had been focused on tackling corruption in the former Belgian colony. "This is the beginning of national reconciliation".
Felix Tshisekedi was declared the victor of the country's long-delayed elections.
Some Congolese have said Tshisekedi lost support by splitting the opposition.
Spokesmen for Shadary and the other top opposition candidate, Felix Tshisekedi, have indicated that their candidates won.