Security forces have deployed near the residence of opposition presidential candidate Martin Fayulu in Democratic Republic of Congo.
He insists he won last month’s election and is due to file a court petition. Mr Fayulu has demanded a manual recount.
But the electoral commission declared another opposition leader, Felix Tshisekedi, the winner.
Mr Fayulu accuses Mr Tshisekedi of striking a power-sharing deal with the outgoing president, Joseph Kabila.
The electoral commission also said the pro-Kabila coalition had won a majority of parliamentary seats.
Several Western governments and the influential Roman Catholic Church in DR Congo have expressed surprise and concern at the declared results.
Mr Kabila has been in office for 18 years and the result, if confirmed, would create the first orderly transfer of power since independence from Belgium in 1960.
What does Fayulu say?
Dozens of Fayulu supporters had gathered outside his residence in the capital, Kinshasa, to chant slogans against Mr Kabila and Mr Tshisekedi.
However they fled inside the building when the security forces arrived on Saturday, Reuters news agency reports.
Speaking earlier to BBC Africa editor Fergal Keane, Mr Fayulu said he would challenge the declared result in the constitutional court.
“I will do whatever is possible for me to do to get the truth because the Congolese want change,” he said.
Mr Fayulu admitted 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 any chance to say he had not followed the law.
“Felix Tshisekedi has been nominated by Mr Kabila to perpetuate the Kabila regime. Because today the boss is Kabila,” Mr Fayulu said.
“Mr Kabila cannot stay and make an arrangement with someone who will not have any power… Mr Tshisekedi knows himself that he did not win.”
Mr Fayulu said he feared there would be violence if the electoral commission did not give the true figures “polling station by polling station” and that it was the right of all Congolese to demonstrate according to the law.
Has there been any violence?
Thousands of supporters of Mr Tshisekedi took to the streets to celebrate but those who backed Mr 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 Mr Tshisekedi has broad support, was mainly celebrating.
UN Secretary General Antonio Guterres has urged all sides “to refrain from violence” and widespread unrest has not yet been reported.
What was the result of the election?
According to the National Electoral Commission (Ceni), Mr Tshisekedi received 38.5% of the vote in the 30 December election.
The full results were:
- Felix Tshisekedi – 7 million votes
- Martin Fayulu – 6.4 million votes
- Emmanuel Shadary – 4.4 million votes
Turnout was reported to be 48%.
What could happen next?
Candidates must file any appeal to the constitutional court within 48 hours of the announcement of the provisional results.
Judges will then have seven days to deliberate.
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 call fresh elections.
The constitutional court has never overturned results before, and some think most of its judges are close to the ruling party.
If Mr Tshisekedi were confirmed as the winner, he would be expected to be inaugurated within ten days.
Why DR Congo matters: