Earlier, the NPP said it had "enough concrete evidence" to prove that Mr Akufo-Addo had won the election.
Nana Akufo-Addo, the candidate for the New Patriotic Party (NPP), garnered 47.7% of the vote, according to the commission.
The US has just put up its latest spacecraft, called NPP, which will soon be feeding data into our forecasts.
The discrepancy prompted hundreds of NPP supporters to hit the streets Sunday to protest what their leaders describe as election fraud.
Mr Akufo Addo was a founding member of the NPP and served as foreign minister in 2002-7 under President John Kufuor.
However, the NPP lacked logistics and money to pay for media advertising campaigns, and Mr Kufuor is unconvincing as a public speaker.
But in 2008 he distanced himself from Mr Rawlings, even drawing criticism from his former boss for being too gentle with the NPP.
"To accept this result is to discredit democracy in Ghana and, in the process, distort the process of democratisation in Africa, " the NPP said.
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 NPP filed a petition over the election result at the Supreme Court in late December, saying it had found irregularities including unregistered voters casting ballots.
Ghana's privately-owned Joy FM radio station reports that about 40 NPP supporters besieged the home of former President Kufuor to prevent him from attending the ceremony.
In contrast to the advertising billboards of his rival NPP candidate, Mr Atta Mills began his campaign with modest-sized signboards bearing the slogan "I Believe in Ghana".
While a lot of night-oil has gone into the NPP's manifesto of prescriptions for the ailing economy and social sectors, it was largely based on pledges, and plays on hope.
"The NPP has enjoyed higher international prices for cocoa and gold, and more manageable petroleum prices, and yet the price of petrol has doubled, " Rojo Mettle-Nunoo, a Mills campaign aide, points out.
The NPP leader said it had not been an easy decision to go ahead with the challenge, but the evidence submitted was "mind-blowing and came as a shock even to sceptics in the party".
The candidates who participated in this High Level event include current President and flag-bearer of the ruling National Democratic Congress (NDC), John Dramani Mahama, and his main challenger and leader of the opposition New Patriotic Party (NPP), Nana Dankwa Akufo-Addo.
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