Ghana's President John Mahama called opposition Nana Akufo-Addo on Friday to concede defeat and congratulate him on victory in the country's hotly contested election, a party official said.

"Yes he has conceded defeat," George Lawson of Mahama's New Democratic Congress (NDC) party told AFP, after Wednesday's nail-biting poll, seen as a test for a country generally viewed as a beacon of stability in west Africa.