Mr Kabila enjoys most of his support in the east of the country
The country's electoral commission announced Mr Kabila won 58.05% of the vote, ahead of ex-rebel leader Jean-Pierre Bemba who got 41.9%.
A member of Mr Bemba's camp has vowed to challenge the vote by legal means.
But peacekeepers have deployed extra troops in the capital, Kinshasa - a Bemba stronghold, in case of trouble.
Peace must reign in every corner of the country
President Joseph Kabila
Mr Kabila told the BBC the country should remain quiet because a new page of its history had just been turned.
The BBC's Mark Doyle says Mr Kabila, who obtained relatively few votes in the capital and in the north of the country, will face major difficulties ruling DR Congo.
Fraud accusations
Kinshasa is quiet and the streets empty, says the BBC correspondent there, Arnaud Zajtman.
But in Lubumbashi, DR Congo's second town and a stronghold of Mr Kabila, people are celebrating.
"Peace must reign in every corner of the country. Long live democracy. Long live the new DR Congo," Mr Kabila said in a televised address after the results were announced on Wednesday night.
"I ask you tonight to remain united and to live in fraternity and tolerance."
Final results were not expected until Sunday, but the electoral commission announced the official verdict earlier, despite objections already lodged by Mr Bemba's team.
The result must now be upheld by the supreme court.
Electoral commission head Apollinaire Malu Malu called for candidates to respect election rules after the Bemba coalition said their candidate had received more than 50% of the vote and that victory "was being stolen from the Congolese people".
HAVE YOUR SAY We understand that Kabila himself will not bring peace - he needs the help of the people
Lwango Tchomba, South Africa
Send us your comments
Mr Bemba has not yet commented on the results, but correspondents say some in Bemba'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 quotes 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."
Armed clashes
An analyst who wished to remain anonymous told the BBC that there were serious questions about the validity of some ballot papers, especially a large number of votes cast by voters outside their home areas.
At least four people were killed in Saturday's clashes
Our correspondent adds that there were big question marks over Mr Kabila's tactics ahead of polling, when soldiers intimidated voters and he used the national TV station as a propaganda tool.
The Carter Center mission said they were confident the results announced by the CEI were consistent with those obtained in the polling stations, but they said it was premature to draw firm conclusions about the overall integrity of these election results.
The vote has been the first following DR Congo's five-year conflict.
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.
President Joseph Kabila of the Democratic Republic of the Congo, when meeting with United States Deputy Secretary of Defense Paul Wolfowitz at the Pentagon in 2003.
Incumbent President Joseph Kabila has been declared the winner of the 2006 Presidential elections by the Independent Electoral Commission of the Democratic Republic of Congo. 

Chairperson of the electoral commission, Apollinaire Malu Malu, announced that Kabila polled 58 percent of the vote to his rival Jean-Pierre Bemba's 42 percent, in the run-off vote held on October 29. 

Kabila has called for calm following the announcement by the election commission. 

Bemba has released a statement saying he does not accept the declared election result. Bemba and his supporters have previously disputed the provisional results announced by the commission. 

Kabila is the world's youngest head of state, getting the title as leader of Congo in 2001, after his father, Laurent-Desiré Kabila was assassinated in 2001. 

Election officials have rejected claims of fraud. International observers monitoring the polls have said that despite some problems, the polling was largely free and fair. 

Kinshasa is tense amidst fears that violence between the two leaders' armed supporters may break out. The U.N. peacekeeping force has stepped up security, deploying tanks on the streets of the capital.