Martin Fayulu, the defeated opposition candidate in DR Congo’s presidential election, has vowed to challenge the result in court.
Felix Tshisekedi, another opposition candidate, was declared the winner amid accusations of a power-sharing deal with the outgoing president.
Several deaths and injuries have been reported in the wake of the results.
The election was to choose a successor to Joseph Kabila, who has been in office for 18 years.
The result if confirmed would create the first orderly transfer of power since independence from Belgium in 1960.
The influential Catholic Church, which posted 40,000 election observers, said the result did not match its findings.
Thousands of supporters of Felix Tshisekedi took to the streets to celebrate but those who backed Martin Fayulu also came out in protest.
Violent scenes were reported in Kikwit, where at least two policemen and two civilians were said to have been killed.
There were reports of several hundred students protesting against the result and being dispersed by tear gas in the town of Mbandaka.
Protests were also reported in Kisangani but the south, where Felix Tshisekedi has broad support, was mainly in celebration.
UN Secretary General Antonio Guterres has urged all sides “to refrain from violence” and widespread unrest has not yet been reported.
According to the National Electoral Commission (CENI), Felix Tshisekedi received 38.5% of the vote in the December 30 election.
The full results were: Felix Tshisekedi – 7 million votes; Martin Fayulu – 6.4 million votes and Emmanuel Shadary – 4.4 million votes.
Turnout was reported to be 48%.