Ghana's main opposition party has filed a petition at the Supreme Court to challenge President John Mahama's victory in this month's election.
The New Patriotic Party (NPP) rejected the results of the 7 December poll, alleging fraud.
The election commission said Mr Mahama had secured 50.7% of votes, enough to avoid a run-off against NPP candidate Nana Akufo-Addo with 47.7% of the vote
International election observers described the poll as free and fair.
Mr Akufo-Addo lost the 2008 presidential run-off by one percentage point, but accepted the result.
Ghana is regarded as one of Africa's most stable democracies.
The NPP said it had waited to file its challenge in court until it had analysed the data from 26,000 polling stations.
It said it had now found irregularities such as cases of over voting and instances when people not registered by the new biometric finger-printing system were able to vote.
The BBC's Sammy Darko in the capital, Accra, says the party has calculated that there were 1.34 million extra votes cast, which if withdrawn from the final tally would make Mr Akufo-Addo the winner.
"We are ready to concede that in an election that involves more than 11 million voters there might be mistakes," said Mr Akufo-Addo after the petition was filed at Supreme Court in Accra.
"But why are the arithmetic mistakes so very often in favour of the NDC presidential candidate, John Dramani Mahama?"
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".
"This case is seeking to deepen our democracy by strengthening the institutions that are mandated by our constitution to superintend the electoral process," Mr Akufo-Addo said
"One, by ensuring that the electoral commission is accountable to the people of Ghana; and two, the Supreme Court is seen by all as the ultimate arbiter of electoral grievances and disputes."
Mr Mahama, from the National Democratic Congress (NDC) party, was Ghana's vice-president until the unexpected death of President John Atta Mills in July catapulted him into office.
Johnson Asiedu Nketia, the NDC's secretary general, said the governing party did not believe the Supreme Court would rule against them as the elections had been the country's most transparent ever.
"We don't have any shred of doubt in our minds that President Mahama has been the choice," Mr Asiedu Nketia said.
Our correspondent says the case will not affect Mr Mahama's swearing-in ceremony on 7 January.
According to the constitution, the election commission's announced winner remains the legal leader until the Supreme Court rules otherwise.
The election challenge case is expected to be heard in three weeks, our reporter says.