(CNN) -- Ghana's election commission announced Sunday night that the West African nation's president won re-election, though the main opposition party says it has "credible evidence" the results were manipulated.
In a statement streamed live on the Internet, Electoral Commission Chairman Kwadwo Afari-Gyan declared "John Dramani Mahama president-elect" after securing 50.7% of the vote. Nana Akufo-Addo, the candidate for the New Patriotic Party (NPP), garnered 47.7% of the vote, according to the commission.
"We must celebrate together as Ghanaians and refrain from anything that will derail the peace and unity we have enjoyed over the years," Mahama told supporters after the result was announced.
But reiterating claims made earlier that the vote had been "manipulated," the New Patriotic Party issued a statement it has "credible evidence (that) undermines the integrity of the electoral process and the results."
"Substantial discrepancies have been discovered from results from coalition centers when compared with the official tally," the party said on its website. "Considering the closeness of the polls, this error is very significant and goes to the heart of the credibility of the results."
Ghanaians voted Friday and Saturday for a president and 275 parliamentary seats. According to the government's website, nearly 11 million citizens -- or about 80% of registered voters -- participated. Mahama is a member of the National Democratic Congress (NDC).
Preliminary results reported by Ghanaian news outlets indicated Mahama was narrowly leading Akufo-Addo, the son of a former president.
That contradicted what NPP General Secretary Kwadwo Owusu Afriyie had said Saturday night, when he claimed Akufo-Addo had won the presidency with 51% of the vote.
The discrepancy prompted hundreds of NPP supporters to hit the streets Sunday to protest what their leaders describe as election fraud. Dressed in party T-shirts, they marched to the nation's Electoral Commission office chanting, "We want peace," according to the state-run Ghana News Agency.
"The results have been manipulated on so many levels," party spokesman Yaw Buaben Asamoa said.
Jake Obetsebi-Lamptey, the NPP's chairman, had called -- without success -- for Sunday's official announcement of results from the election commission to put off until "all the allegations have been investigated."
He said tens of thousands of votes were added to Mahama's tally "following collusion" between National Democratic Congress and election officials, Ghana News Agency reported.
The NPP has asked Ghana's electoral commission to recount the vote and conduct an audit of voting machines "to help establish the credibility and accuracy of this year's presidential election.
"This in my view would assist considerably to allay public anxiety, which is growing hour by hour and due to the announcements being made in the Ghanaian media," Obetsebi-Lamptey wrote in a letter released by the party. "It would also obviate any legal and protracted judicial proceedings on the issues and permit the resolutions of our concerns promptly, to enable due declaration to be made."
NDC General Secretary Johnson Asiedu Nketia told reporters his party was not yet responding to the NPP claims, expressing faith well before Sunday night's announcement by Ghana's election commission that Mahama was "moving towards victory."
Unlike its neighbors, Ghana has held successful elections and power transfers since 1992 without descending into bloody chaos. The electoral commission initially said it expected to declare the winner within 72 hours after polls closed Friday. Passions are running high, and the president urged candidates to ensure that their supporters avoid incitement.
"Ghana has organized five previous successful elections, and there should not be any reason why this year's election should not be successful," Mahama said in a statement.
Voting was extended into Saturday in areas where election materials arrived late or glitches with a new biometric identification system caused delays. Observers from the Economic Community of West African States noted some discrepancies and problems with voting procedures but described the vote as "generally peaceful and transparent."
Ghana is one of Africa's fastest-growing economies. It is the world's second-largest cocoa producer, after Ivory Coast, and the continent's second biggest gold miner, after South Africa, according to the United Nations.
But critics say that despite the rich resources that bring billions of dollars annually, the wealth is not trickling down to the rural poor who live on the land where the gold is mined.
Ghana was among the first African countries to gain independence from the British, breaking loose in 1957. It endured a series of coups before Lt. Jerry Rawlings took power in 1981. A decade later, it transitioned to a stable democracy with multiparty elections.