Ghana's main opposition party says it is considering legal action to annul President John Mahama's victory in Friday's fiercely contested election.
NPP chairman Jake Obetsebi Lamptey told the BBC it would decide on Tuesday whether there was "robust" evidence to prove the result had been manipulated.
The election commission said Mr Mahama had secured 50.7% of votes, compared to NPP candidate Nana Akufo-Addo's 47.7%.
Ghana is regarded as one of Africa's most stable democracies.
African Union (AU) commission chairman, Benin's President Thomas Yayi Boni, has flown to Ghana for talks with Mr Mahama and Mr Akufo-Addo, Ghana's privately-owned Joy FM radio station reports on its website.
The AU and other observers have declared the election free and fair.
Mr Lamptey told the BBC's Focus on Africa programme that the election has been marred by "widespread abuse" and the result had been "manipulated" in Mr Mahama's favour.
He said the New Patriotic Party's (NPP) top leadership body, the national executive committee, would meet on Tuesday to consider its options.
"They will hear from lawyers about whether the evidence is robust enough to go to the Supreme Court with.... They will then make a decision on what the next step should be," Mr Lamptey said.
Electoral commission chairman Kwado Afari-Gyan said the NPP had failed to offer proof to support its allegations.
It could challenge the result in court if it wanted to, he said.
After talks with Mr Mahama, Mr Boni said he hoped that the opposition's allegations of fraud would not trigger violence and the dispute would be resolved through legal channels, Joy FM reports.
Mr Boni is still due to meet Mr Akufo-Addo, Joy FM reports.
Mr Lamptey told the BBC the NPP would not resort to violence to overturn the result.
"You don't improve democratic practise by burning the country," he said.
The election was also declared free and fair by the regional body, the Economic Community of West African States (Ecowas) and a local group, the Coalition of Domestic Election Observers (Codeo).
"There were hiccups but not such that would grossly undermine the result of the election," said former Nigerian President Olusegun Obasanjo, who led the Ecowas observer team to Ghana, AP news agency reports.
In a statement, Codeo said the result was a "generally accurate reflection" of the support for Mr Mahama and Mr Akufo-Addo.
"Codeo advises all the presidential election contestants and their supporters as well as the general public to place confidence in the electoral commission's official presidential election results," it said.
In his first address after being declared the winner, President Mahama urged "all leaders of all political parties to respect the voice of the people".
He added: "The voice of the people is the voice of God."
Police in the capital Accra fired tear gas to disperse opposition protesters from outside the electoral commission's offices on Sunday evening.
Tanks guarded the electoral commission and roads around the offices were barricaded by police as the results were announced.
The turnout had been about 80%, the commission said.
The NPP rejected the results soon after they were released.
"To accept this result is to discredit democracy in Ghana and, in the process, distort the process of democratisation in Africa," the NPP said.
"The ruling NDC conspired with certain EC staff in constituencies across the country to falsify the election results and thereby abuse the mandate of the people of Ghana," the party said.
The party said in a statement on its website that in the northern Savelugu constituency, 31,165 votes had been "falsely declared" for Mr Mahama when he had, in fact, obtained 21,165 votes.
In another case, 15,000 votes were "illegally added" to Mr Mahama's tally in Greater Accra's Dome Kwabenya constituency, it said.
Mr Mahama's presidential adviser Tony Aidoo said the accusations were baseless.
Mr Akufo-Addo lost the 2008 presidential poll by one percentage point, but accepted the result.
Glitches with a new biometric finger-printing system meant that voting continued into Saturday in some parts of the country.
Mr Mahama was Ghana's vice-president until the unexpected death of President John Atta Mills in July catapulted him into office.