However, Shadary performed badly in the presidential election: he won only 4.4 million votes out of 46 million registered voters coming a distant third to the opposition candidates. The Democratic Republic of Congo's influential Catholic church also said Tshisekedi's victory did not tally with data collected by its own monitors.
The runner-up, opposition candidate Martin Fayulu, has told the BBC he will mount a legal challenge against the official vote count.
The official results are "highly implausible", said Pierre Englebert, a USA expert on African politics and a senior fellow at the Atlantic Council, in an analysis published on Thursday.
Fayulu says he will do whatever is possible for him to get the truth because the Congolese want change.
There was widespread speculation by analysts and politicians that Mr. Kabila negotiated a backroom power-sharing deal with Mr. Tshisekedi, allowing him to become president while Mr. Kabila continues to control key sectors such as the military and security agencies.
There are fears the result could trigger unrest, with at least two people killed in the western city of Kikwit on Thursday. The country of more than 80 million people, which has suffered wars and poverty for decades, is seen as crucial to stability in central Africa.
Why is the result controversial?
Public and diplomatic pressure mounted Monday (8 January) on the Democratic Republic of Congo to announce the outcome of 30 December elections to replace long-term leader Joseph Kabila. "No-one could imagine the scenario where an opposition candidate could be victorious".
He also noted the "importance of President Joseph Kabila's decision to abide by his constitutionally mandated term limits and transfer power to a successor".
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For example, an investigation in 2013 found that a staff member had made "inappropriate, sexual statements" about four clients. The birth has triggered a police investigation and reviews by state agencies.
Tshisekedi has pledged to work closely with Kabila.
Mr. Mahamat who took cognizance of the provisional results of the Presidential election in the DRC, as announced by the Independent National Electoral Commission congratulated the people, political actors, civil society and relevant national institutions for the conduct of elections deemed satisfactory by African Union Election Observer Mission led by Dioncounda Traore, Mali's former President. Mr Tshisekedi's spokesman, Louis d'Or Ngalamulume, said there was "never any deal".
The Catholic Church's bishops conference said last week that it knew the identity of the victor and demanded that CENI publish accurate results. It said it was awaiting clarification from worldwide observers and urged all sides to refrain from violence.
Vote tallies by Congo's Catholic church (CENCO) showed second-placed opposition candidate Martin Fayulu as victor, according to two diplomats briefed on the findings.
Election commission (CENI) president Corneille Nangaa, speaking around 3 a.m. (0200 GMT) at a news conference that appeared timed to avoid any immediate reaction in the streets, said Tshisekedi had won with 38.57 percent of the more than 18 million ballots cast.
But a court challenge to the results could spin the country into chaos, observers warned.
The eastern DR Congo is a sanctuary for a plethora of rebel and militia groups that menace both the civilian populations inside and neighbouring countries.
The security forces have used live ammunition as well as tear gas and beatings during previous protests.