There have been appeals for calm in the Democratic Republic of Congo following the victory of President Joseph Kabila in disputed elections.
Main opposition candidate Etienne Tshisekedi has rejected official results and declared himself the winner, raising fears of violence.
Mr Tshisekedi along with the EU, the US, Britain, France and former colonial power Belgium appealed for calm.
Riot police are patrolling the capital, Kinshasa, and gunshots have been heard.
The city, in the west of the country, is an opposition stronghold and columns of smoke were seen rising over districts backing Mr Tshisekedi as groups of young men burned tyres.
On Friday evening, election commission chief Daniel Ngoy Mulunda announced that President Kabila had gained 49% of the vote against 32% for Mr Tshisekedi.
Many shops and stalls in Kinshasa's markets have been closed for most of the week as the country awaited the results, which were delayed by several days.
Meanwhile, in areas loyal to President Kabila, mainly in the east of the country, residents cheered and supporters staged victory parades.
"I reject these results, and in fact I see them as a provocation against our people," said 78-year-old Mr Tshisekedi.
"It is scandalous and vulgar. We have done our own calculations and I received 54% to Kabila's 26%. His term is finished. I am the president."
Mr Tshisekedi later appealed to his supporters to "stay calm and peaceful".
However, he added that he was waiting to see if diplomatic efforts would change the situation.
The army says it has about 20,000 soldiers on standby in Kinshasa. The atmosphere in the city is said to be tense.
UN Secretary-General Ban Ki-moon called for "any differences regarding the provisional results of the polls to be resolved peacefully through available legal and mediation mechanisms".
The French Foreign Ministry appealed for peace, saying: "France calls on all Congolese political players to show restraint and a spirit of responsibility."
US State Department spokeswoman Victoria Nuland said Washington was calling on DR Congo's leaders and their supporters "to act responsibly, to renounce violence, to resolve any disagreements they might have through peaceful dialogue".
The announcement of the results had been delayed since Tuesday, with election officials blaming logistical problems.
Four other candidates have said the election was rigged and should be annulled.
International observers said the vote was flawed but stopped short of calling it fraudulent. Most said the irregularities were not enough to change the outcome.
Deadly clashes marred the period leading up to the election and thousands of foreigners and Congolese have fled the country for fear of further violence.
Mr Kabila, 40, has been president since 2001 following the death of his father, Laurent.
In 2006 he won the first elections since the end of a five-year conflict and is due to be sworn in on 20 December for his second term.
But his victory must first be confirmed by the supreme court.
He enjoys greater popularity in eastern areas, where his origins lie and where he is credited with helping to end the war.
However, he is less popular in the west, partly because he is not fluent in the local Lingala language and because some see him as representing foreign interests.
Mr Tshisekedi has said he has no intention of taking an election dispute to the court, which he regards as "Kabila's private institution".
Earlier this year, the constitution was amended so that the candidate with the most votes would win the election, removing the need for a second round.
The BBC's Thomas Hubert in Kinshasa says the move succeeded in dividing the opposition.
But it means Mr Kabila is bound to face legitimacy challenges as he has been re-elected with less than 50% of the vote, amid widespread suspicion of electoral fraud and with very little support in the west.
Although DR Congo is rich in minerals such as gold and diamonds, years of conflict and mismanagement mean it recently came bottom of a survey of living standards around the world.