The Democratic Republic of Congo President Joseph Kabila has rejected claims that he won elections through widespread rigging.
The Carter Center observer group said the results "lack credibility", while the Catholic archbishop of Kinshasa said they did not reflect "the truth".
Mr Kabila denied this but admitted that "mistakes" had been made.
Opposition leader Etienne Tshisekedi claimed victory for himself. Several people were killed in weekend protests.
After days of delays, Friday's official results which gave Mr Kabila 49% of the vote against 32% for the 78-year-old Mr Tshisekedi.
At a news conference in the capital, Kinshasa, Mr Kabila, 40, denied that the results lacked legitimacy.
"The credibility of these elections cannot be put in doubt," he said.
"Were there mistakes? Definitely, but [the US-based Carter Center] has definitely gone far beyond what was expected."
He pointed to his own disappointing scores in the eastern provinces as proof that the election process had been transparent and said these elections were "far better" than those in 2006, when he was first elected.
Mr Kabila said he was not surprised by Mr Tshisekedi's declaration of himself as president, but he would press ahead with the task of governing DR Congo and was confident that the economy would achieve double-digit growth in the next two or three years.
"We don't have a crisis in this country... We're going to stay calm and continue with the day-to-day activities of the state," he said.
The people of this country don't want another turmoil, another conflict, another crisis. That's the last thing we need in the Congo."
The opposition have announced plans to hold protest marches after rejecting Mr Kabila's victory.
"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 other opposition candidates have said the election was rigged and should be annulled.
DR Congo, a country two-thirds the size of western Europe but with hardly any tarred roads or other basic infrastructure, is trying to recover from the 1998-2003 war which claimed an estimated four million lives.
International Criminal Court (ICC) chief prosecutor Luis Moreno-Ocampo warned last week that renewed conflict would be a ticket to The Hague, not power.
Mr Kabila's victory in the last election in 2006 led to street battles between government security forces and militias allied to losing candidate, Jean-Pierre Bemba.
Mr Bemba is now on trial at the International Criminal Court for alleged war crimes in neighbouring Central African Republic.
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.
The Catholic archbishop of Kinshasa, Laurent Monsengwo, said the results "comply with neither the truth nor justice".
The BBC's Thomas Hubert in Kinshasa says the Roman Catholic Church was one of the main election observer groups, with some 30,000 monitors around the country.
Archbishop Monsengwo said the results should be challenged in the Supreme Court.
"[The court] is called by all Congolese people to say what is really right."
However, Mr Tshisekedi said he would not mount a legal challenge against the results.
He described the results as a "provocation" and "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.
Mr Tshisekedi is hopeful that the international community can mediate a solution to the crisis, his spokesman says.
Mr Kabila has been president since 2001 following the assassination of his father, Laurent.
He is due to be sworn in on 20 December for his second term.
But his victory must first be confirmed by the Supreme Court.