(Reuters) - Ghana's main opposition party said on Sunday the country's presidential election had been rigged, raising concerns of unrest in a nation seen as a bulwark of democracy in an unstable region. The New Patriotic Party called on the electoral commission to delay announcing results hours after local media projected its candidate Nana Akufo-Addo had narrowly lost to incumbent John Dramani Mahama. "These results cannot be forced down the throat of the people of Ghana," NPP chairman Jake Obetsebi-Lamptey told reporters at a news conference in Accra. The poll is 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. Local news outlet Joy News used counts from polling stations to project a narrow Mahama victory in the first round vote. Obetsebi-Lamptey said he had evidence of electoral workers conspiring to rig tallies and added the party had sent a formal letter of complaint to the election commission asking for an audit before full results are released. Election commissioner Kwadwo Afari-Gyan told Reuters he was not yet aware of the NPP complaint, and declined to say when the body would announce a winner. At least two hundred supporters of Akufo-Addo, wearing red headbands and shouting "We want peace, we want SHS", faced off with riot police and soldiers near the Election Commission building. SHS refers to Akufo-Addo's promise to provide Ghanaians with free education through senior high school. Akufo-Addo, a British-trained lawyer, has also criticised the ruling party for failing to root out government graft. Mahama served as prime minister for former leader John Atta Mills and took over the presidency in July after Mills' death from illness. He has vowed to use rising oil revenues to jumpstart development, create jobs, and combat poverty. But in a country where campaign messages rarely influence voting choices, many believe most of Ghana's 14 million voters cast their ballots based on ethnic, social or regional ties. "We have a fair idea what the outcome of the elections will be," Mahama said at his house in a leafy suburb of Accra on Sunday. "But as a law-abiding political party, we shall wait for the electoral commission to make an official declaration." A run-off is possible December 28 if no candidate wins an outright majority. Ghanaians are also electing a parliament, in which Mahama's party has enjoyed a slim majority. An oil-driven economic boom has brought more wealth to the country, but also fears that it could suffer the graft and turmoil that often plagues energy-rich developing nations. TROUBLED POLLS Voting was plagued by delays after hundreds of newly-introduced electronic fingerprint readers - used to identify voters - failed on Friday and forced some polling stations to reopen on Saturday to clear the backlog. Ghana television stations aired long infomercials on Sunday, between election updates, showing clips of wars that have erupted in neighbouring countries interspersed with testimonials from Ghanaians about the importance of maintaining peace. "This election has been hard, but we must remember Ghanaians are one and we must love each other and remain peaceful," said Wellington Dadzie, 69, a former soldier who lives on the outskirts of the capital Accra. Ghana has had five peaceful and constitutional transfers of power since its last coup in 1981. Its residents like to say "Ghana in peace, not in pieces". Neighbouring Ivory Coast tipped into civil war last year after a disputed 2010 poll and regional neighbours Mali and Guinea-Bissau have both suffered coups this year. Oil production in Ghana - which is also a big cocoa and gold producer - started two years ago and oil field operator Tullow Oil says it expects to boost output further in 2013. "These elections are important not just to Ghana but for the growing number of states and actors seeking to benefit from increasing confidence in Africa," said Alex Vines, Africa Research Director at Chatham House.