Losing Democratic Republic of Congo presidential candidate Martin Fayulu's campaign said on Friday its tallies showed he had easily won the election ahead of the official winner announced by the electoral commission.
Campaign official Fidele Babala told reporters Mr Fayulu won 61.51 percent of the vote, while president-elect Felix Tshisekedi took 18.86 percent and ruling coalition candidate Emmanuel Ramazani Shadary received 18.49 percent.
The vote had been intended as Congo's first democratic transfer of power in six decades, but instead threatens to reawaken violence in the huge and tumultuous nation where millions have died during civil wars since the 1990s.
"When you know you are in the right, you are not allowed to remain home," Mr Fayulu said, urging supporters to "rise up".
Though pre-election polls predicted a landslide for Mr Fayulu, a businessman and former manager at US oil giant Exxon Mobil, the national election board (CENI) said he lost to another opposition leader, Mr Tshisekedi, 55.
The official results put Mr Tshisekedi at 7 million votes, Mr Fayulu at 6.4 million, and Mr Shadary at 4.4 million.
Mr Fayulu's supporters say authorities rigged the result in a deal to protect members of Joseph Kabila's outgoing administration and maintain his influence over security forces.
The influential Catholic Church has also rejected the official result based on tallies by its 40,000-strong observer team. France and former colonial power Belgium also expressed doubts.
Mr Fayulu's camp said it would take its case on Saturday morning to Congo's highest court. It has asked CENI to publish results from every polling station.
"We know 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," Mr Fayulu told the BBC.
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.
There have been isolated incidents of post-election violence in the nation of 80 million.
Police confronted opposition protesters in the eastern city of Goma on Friday, killing at least one person, a Reuters witness said.
In the northern city of Kisangani, police and army responded to student protests, and in nearby Mangobo a local ruling party office was set on fire, according to an internal UN report seen by Reuters.