An opposition presidential candidate in the Democratic Republic of the Congo appealed the election results of last month.
Martin Fayulu insists that he won the election and demanded a manual recount, but the electoral commission declared rival Felix Tshisekedi the winner.
Security forces have settled near Mr. Fayulu.
He accuses Mr Tshisekedi, also an opposition candidate, of making an agreement with outgoing President Joseph Kabila.
Announcing the appeal, Mr Fayulu's lawyer Feli Ekombe, quoted by AFP, said: "The petition calls for the cancellation of the results. He has proclaimed Felix Tshisekedi President of the Republic." [Friday] 1
The Electoral Commission also said that the pro-Kabila coalition had won a majority of parliamentary seats.
Several Western governments and the influential Roman Catholic Church in the Democratic Republic of the Congo expressed surprise and concern at the declared results.
Mr Kabila has been in office for 18 years, and the result, if confirmed, would be the first proper transfer since independence from Belgium in 1960. What does Fayulu say?
Dozens of Fayulu supporters gathered outside his home in the capital Kinshasa to sing slogans against Messrs. Kabila and Tshisekedi.
However, when the security forces arrived on Saturday, they fled into the building, reports Reuters.
In an earlier conversation with Fergal Keane, publisher of BBC Africa, Fayulu said he would contest the outcome before the Constitutional Court.
"I will do everything that is possible for me to know the truth, because the Congolese want to change," he said.
Mr. Fayulu admitted that such a challenge would have little chance of success as the court was "composed" of Kabila's people, "but he said he did not want to give his opponents a chance to say he had not obeyed the law."
"Felix Tshisekedi was nominated by Mr. Kabila to perpetuate the Kabila regime because the boss today is Kabila," Mr. Fayulu said.
"Mr. Kabila can not stay and make an agreement with someone who has no power … Mr. Tshisekedi himself knows that he did not win."
Mr Fayulu said he feared there would be violence if the electoral commission proceeded did not give the true numbers "polling station by polling station" and that it was the right of all Congolese to demonstrate according to the law.
Was there violence?
Thousands of followers of Mr. Tshisekedi took to streets that were celebrated, but those who supported Mr. Fayulu also protested.
In Kikwit, violent scenes were reported in which at least two policemen and two civilians were killed.
There were reports of several hundred students protesting against the result and tear gas in the city of Mbandaka were scattered.
Protests were also reported i In Kisangani, the south, where Mr. Tshisekedi finds broad support, celebrated mainly.
UN Secretary-General Antonio Guterres has called on all sides to "refrain from violence" and has not yet reported widespread riots.
What was that? Election result?
According to the National Electoral Commission (Ceni), Mr. 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
- Emmanuel Shadary – 4.4 million votes
The turnout was 48%.
What could happen next?
Candidates must appeal to the Constitutional Court within 48 hours of the announcement of the preliminary findings.
The judges will then have seven days to discuss it.
The constitutional expert Jacques Ndjoli told the BBC There were three possible outcomes: The court could confirm Mr Tshisekedi's victory, order a recount, or cancel the results altogether and demand new elections.
The Constitutional Court has never before reversed the findings, and some believe that most of its judges are close to the ruling party.
If Mr. Tshisekedi was confirmed victorious, he would probably be opened within 10 days.
Why DR Congo matters: