A team of international observers has said irregularities in DR Congo's presidential election were so serious that the results "lack credibility".
The Carter Center highlighted numerous problems in the vote-counting process.
The official results announced on Friday gave President Joseph Kabila 49% of the vote against 32% for opposition leader Etienne Tshisekedi.
Mr Tshisekedi says he won the election and is the country's president. He has urged supporters to stay calm.
The opposition have announced plans to hold protest marches next week.
"We insist that the protests will be non-violent. The population know this may be a long, long walk but they are ready for it," opposition spokesman Albert Moleka told Reuters news agency.
Four people died in post-election violence after the results came out.
Four other opposition candidates have said the election was rigged and should be annulled.
In a statement, the Carter Center, which had 26 teams of observers monitoring the elections, pointed to differences in the vote count between areas where Mr Kabila had strong support and areas that favoured Mr Tshisekedi.
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", the Center said.
Meanwhile in Kinshasa, where Mr Tshisekedi has strong support, results from nearly 2,000 polling station stations were lost - roughly a fifth of the city's total.
The Center said the violations it had documented does not mean "the final order of candidates is necessarily different" from official results.
But it said that further analysis of preliminary results could reveal further discrepancies in the vote counting process.
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", an appeal echoed by the EU, the US, the UK, France and ex-colonial power Belgium.
Mr Tshisekedi is hopeful that the international community can mediate a solution to the crisis, his spokesman says.
UN Secretary-General Ban Ki-moon has called for "any differences regarding the provisional results of the polls to be resolved peacefully through available legal and mediation mechanisms".
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.