BBC
Since the results, there have also been unconfirmed reports of deaths in Mbuji-Mayi and North Kivu.
The main opposition candidate Etienne Tshisekedi has also claimed victory.
The government branded Mr Tshisekedi's move "illegal and irresponsible".
'Impossible turnout' The Carter Center, which had 26 teams of observers monitoring the elections, said the results "lack[ed] credibility".
It criticised irregularities, including the loss of nearly 2,000 polling station results in Kinshasa, where Mr Tshisekedi has strong support.
And it said some constituencies in Katanga province "reported impossibly high rates of 99 to 100% voter turnout with all, or nearly all, votes going to incumbent President Joseph Kabila".
Mr Tshisekedi, along with the EU, the US, the UK, France and ex-colonial power Belgium, has appealed for calm.
Riot police are patrolling the capital, Kinshasa, and gunshots have been heard.
"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 said.
He later appealed to his supporters to "stay calm and peaceful".
Nonetheless Information Minister Lambert Mende said the remarks could spark violence.
"We must firmly condemn the self-declaration by Mr Etienne Tshisekedi," AFP news agency quotes him as telling journalists.
"It's an irresponsible act that violates the laws of the republic."
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".
'Flawed, not fraudulent' The army says it has about 20,000 soldiers on standby in Kinshasa. The atmosphere in the city is said to be tense.
Kinshasa, in the extreme west of the country, is an opposition stronghold and overnight columns of smoke were seen rising over districts backing Mr Tshisekedi as groups of young men burned tyres.
They had been delayed since Tuesday, with election officials blaming logistical problems.
In areas loyal to President Kabila, mainly in the east of the country, residents cheered and supporters staged victory parades.
Alphonse Kasuasua, a civil society leader in East Kasai province where support for Mr Tshisekedi is strong, said security forces were using live bullets to deal with his angry supporters who had taken to the streets.
"I tried to go out this morning but the security forces were firing to force people back into their homes," he said.
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.
Four other candidates have said the election was rigged and should be annulled.
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.
The BBC's Thomas Hubert says shopkeepers in Kinshasa fear looters
Four people have been killed in the DR Congo capital Kinshasa after incumbent Joseph Kabila's victory in presidential polls was announced on Friday.
Police shot dead three looters, with one woman hit by a stray bullet, according to national police chief General Charles Bisengimana.Since the results, there have also been unconfirmed reports of deaths in Mbuji-Mayi and North Kivu.
The main opposition candidate Etienne Tshisekedi has also claimed victory.
The government branded Mr Tshisekedi's move "illegal and irresponsible".
'Impossible turnout' The Carter Center, which had 26 teams of observers monitoring the elections, said the results "lack[ed] credibility".
It criticised irregularities, including the loss of nearly 2,000 polling station results in Kinshasa, where Mr Tshisekedi has strong support.
And it said some constituencies in Katanga province "reported impossibly high rates of 99 to 100% voter turnout with all, or nearly all, votes going to incumbent President Joseph Kabila".
Mr Tshisekedi, along with the EU, the US, the UK, France and ex-colonial power Belgium, has appealed for calm.
Riot police are patrolling the capital, Kinshasa, and gunshots have been heard.
Continue reading the main story
“Start Quote
Alphonse Kasuasua Civil society leader in East KasaiI tried to go out this morning but the security forces were firing to force people back into their homes”
The official results gave President Kabila 49% of the vote against 32% for Mr Tshisekedi.
The 78-year-old opposition leader said the results were a "provocation"."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 said.
He later appealed to his supporters to "stay calm and peaceful".
Nonetheless Information Minister Lambert Mende said the remarks could spark violence.
"We must firmly condemn the self-declaration by Mr Etienne Tshisekedi," AFP news agency quotes him as telling journalists.
"It's an irresponsible act that violates the laws of the republic."
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".
'Flawed, not fraudulent' The army says it has about 20,000 soldiers on standby in Kinshasa. The atmosphere in the city is said to be tense.
Kinshasa, in the extreme west of the country, is an opposition stronghold and overnight columns of smoke were seen rising over districts backing Mr Tshisekedi as groups of young men burned tyres.
Continue reading the main story 
Who is Joseph Kabila?
- 40 years old
- Born in a rebel camp in eastern DR Congo - where he enjoys most of his support
- Spent his childhood in Tanzania
- His father, Laurent Kabila, overthrew long-time ruler Mobutu Sese Seko in 1997
- He first became president after his father's assassination in 2001
- Oversaw the signing of a peace accord in 2002 to end a five-year conflict involving several other nations
- Became DR Congo's first freely elected leader in 2006, winning a run-off poll with 58% of the vote
- His campaign slogan was: "Five building sites of the republic"
- Shies away from public speaking
- Is married with one daughter
"Since last night it has been very bad, we haven't been able to sleep because of the gunfire," Tresor Nkuna, a resident of Kinshasa, told Reuters news agency.
Many shops and stalls in the city's markets have been closed for most of the week as the country awaited the results.They had been delayed since Tuesday, with election officials blaming logistical problems.
In areas loyal to President Kabila, mainly in the east of the country, residents cheered and supporters staged victory parades.
Alphonse Kasuasua, a civil society leader in East Kasai province where support for Mr Tshisekedi is strong, said security forces were using live bullets to deal with his angry supporters who had taken to the streets.
"I tried to go out this morning but the security forces were firing to force people back into their homes," he said.
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.
Four other candidates have said the election was rigged and should be annulled.
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.
Inside DR Congo
SIZE
The Democratic Republic of Congo covers 2,344,858 square km of land in the centre of Africa, making it the 12th largest country in the world.
Mineral Wealth
Transport 
Population
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Mineral Wealth
DR Congo has abundant mineral wealth. It has more than 70% of the world's coltan, used to make vital components of mobile phones, 30% of the planet's diamond reserves and vast deposits of cobalt, copper and bauxite. This wealth however has attracted looters and fuelled the country's civil war.
Transport
Despite the country's size, transport infrastructure is very poor. Of 153,497km of roads, only 2,794km are paved. There are around 4,000 km of railways but much is narrow-gauge track and in poor condition. Waterways are vital to transport goods but journeys can take months to complete. Overcrowded boats frequently capsize, while DR Congo has more plane crashes than any other country.
Population
With an estimated population of 71 million, DR Congo is the fourth most populous country in Africa. Some 35% of the population live in cities and the capital Kinshasa is by far the largest, with more than 8 million inhabitants. DR Congo has around 200 ethnic identities with the majority of people belonging to the Kongo, Luba and Mongo groups.
DATA
Given its size and resources DR Congo should be a prosperous country, but years of war, corruption and economic mismanagement have left it desperately poor. In 2011 it lags far behind in many key development indicators, with average life expectancy increasing by only 2 years since 1980, after a period when it actually fell during the mid 1990s.
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