Democratic Republic of Congo presidential candidate Jean-Pierre Bemba has rejected election results that gave victory to Joseph Kabila.
Bemba has a big following in Kinshasa, the north and west
The electoral commission announced President Kabila won 58.05% of the vote in the run-off poll, ahead of ex-rebel leader Mr Bemba who got 41.95%.
Mr Bemba said he would use all legal channels to contest the result.
Peacekeepers have deployed extra troops in the capital, Kinshasa - a Bemba stronghold - in case of trouble.
Mr Kabila has appealed for calm as his supporters celebrated victory in the landmark election.
The vote has been the first following DR Congo's five-year conflict.
Correspondents say Mr Kabila, who obtained relatively few votes in the capital and in the west of the country, will face major difficulties ruling DR Congo.
'Will of people'
Mr Bemba said the result, announced on Wednesday, did not reflect the truth of the ballot box.
His requests for clarification on some results were not answered, he said.
"I regret to have to say to our people and to the international community that I cannot accept these results," he said in a statement.
"I promise to use all legal means to ensure the will of our people is respected," he added.
The election result has to be confirmed by the Congolese supreme court. Most observers declared the election free and fair.
Correspondents say some in the capital fear a resumption of violence, following August's unrest when Mr Bemba fell behind in the first round of voting.
Earlier, young Bemba supporters stoned passing cars outside his official residence.
But some in the politician's camp are playing down the threat of violence.
"People were expecting a war plan from us, some troubles. But we're not into that," AFP news agency quoted Fidele Babala, a Bemba aide, as saying.
"At the level of the national assembly we have our deputies and we're going to play our role as the opposition."
Forces loyal to the two candidates clashed during the war as well as during the tense election period.
Following violence on Saturday in which four people died, the police arrested 337 homeless people, including 87 children, the government says, blaming them for starting the trouble.
Eyewitnesses say that security forces loyal to the two candidates exchanged gun and mortar-fire.
United Nations observers say the election is the most significant in Africa since Nelson Mandela was elected as South Africa's president in 1994.
The world's largest UN peacekeeping force - 17,000-strong - is in DR Congo, tasked with ensuring security.