* Nana Akufo-Addo has "serious reservations" about poll
* Says party will decide response on Tuesday
* Local observers confirm results
ACCRA, Dec 10 Ghana's main opposition challenger said on Monday his party might challenge election results in the courts after authorities declared incumbent president John Dramani Mahama winner of the Dec. 7 poll.
Ghana's electoral commission said on Sunday Mahama, who replaced former president John Atta Mills after his death in July, had won 50.7 percent of the ballots cast, enough to avoid a run-off against rival Nana Akufo-Addo.
"We have serious reservations about the counting and the declaration of results," Akufo-Addo told Reuters in his office at his residence in Accra.
"If we are going to challenge the results, the main question is; do we have enough evidence to suggest that, materially, the evidence will have affected the outcome?"
"We are not in a position to embrace these results. The obvious option is to go and challenge the results in the courts. The other option is to forego it and make your case to the country," Akufo-Addo said.
The poll is seen as a test of whether Ghana can maintain more than 30 years of stability and progress in a region better known for coups, civil wars and corruption.
A cliff-hanger election in 2008, in which Akufo-Addo lost by less than 1 percent, pushed the country to the brink of chaos, with disputes over results driving hundreds of people into the streets with clubs and machetes.
Akufo-Addo called for calm, saying leaders of his conservative-leaning New Patriotic Party would meet on Tuesday to decide the party's response to the results.
On the wall of his office is a portrait of his father Edward Akufo-Addo, president from Aug. 1970 to Jan. 1972 before he was deposed by a military coup. Outside, there was a dour atmosphere among supporters milling about the yard, some yelling that the election had been stolen.
Voting was fraught with delays after hundreds of newly-introduced electronic fingerprint readers failed on Friday and forced some polling stations to reopen on Saturday to clear the backlog.
But the west African nation's non-partisan Coalition of Domestic Election Observers (CODEO), which deployed more than 4,000 poll watchers, said the vote had been generally free and fair.
CODEO said on Monday its parallel tabulation of results confirmed those declared by the electoral commission.
"The results of the 2012 presidential polls declared by the Electoral Commission are generally an accurate reflection of how Ghanaians voted in the December 7 polls," it said in a statement.
The cocoa and gold-producing nation, which also began pumping oil in 2010, has had five peaceful and constitutional transfers of power since its last coup in 1981.