The runner-up in the Democratic Republic of Congo's presidential election pledged on Friday to launch a formal fraud complaint.
Pre-election polls predicted a landslide win for opposition candidate Martin Fayulu in a vote hoped to bring Congo's first democratic transfer of power since independence.
But in a surprise outcome, the businessman, backed by powerful exiled politicians and ex-militia leaders, lost out to another opposition leader, Felix Tshisekedi, 55.
Fayulu's supporters say authorities rigged the result on behalf of Tshisekedi in a deal to protect members of President Joseph Kabila's outgoing administration and maintain his influence over security forces.
Fayulu told the BBC he would file a challenge at Congo's highest court.
“We know that the Constitutional Court is composed by Kabila's people, but we do not want to give any chance to Kabila and his team to say...you didn't follow the law. We want to do all that we can do to have the clean result,” Fayulu said.
Under law, he has 48 hours to contest the results announced in the early hours of Thursday.
Many Congolese fear the dispute could re-start a cycle of unrest in a country where wars causing hunger and disease have decimated the population in recent decades.
Around the vast nation of 80 million people that is nearly the size of western Europe, there have been isolated incidents of post-election violence.
Police confronted opposition protesters in eastern Goma city on Friday, killing at least one person, a Reuters witness said.
“The people of the Democratic Republic of Congo have waited 58 years for a peaceful democratic transfer of power, and have sacrificed too much for this election result not to reflect their votes cast at the ballot box,” added a bipartisan group of U.S. Senators late on Thursday.
Tshisekedi inherited the leadership of his UDPS party when his father Etienne Tshisekedi died in 2017. But he lacks the experience, clout and firebrand reputation of his father.
Before the vote, Tshisekedi said Kabila had nothing to fear should he come to power. Fayulu, by contrast, is backed by ex-rebel Jean-Pierre Bemba and former governor Moise Katumbi, two of Kabila's fiercest rivals.