The poll raised tensions in the country and challenges by both main parties threatened to mar an election seen as a chance to bolster Africa's democratic credentials after flawed ballots elsewhere.

Results of voting in a final constituency on Friday showed Mills, of the National Democratic Congress (NDC), narrowly defeated Nana Akufo-Addo of the New Patriotic Party (NPP), which lost its parliamentary majority in an election last month.

"I want to assure everybody that I will be president for all. There will be no discrimination," Mills said in a victory speech to thousands of NDC supporters who thronged the streets around his office in the west African country's capital, Accra.

"Our nation is at a crossroads and we must work together to build it peacefully," Akufo-Addo told a news conference. Outside, angry NPP crowds threatened journalists.

Ghana's political stability has attracted growing numbers of foreign investors as it prepares to produce oil in late 2010.

The centre-left NDC which was led by former President Jerry Rawlings until he gave way to Mills in 2000 has promised change after eight years of NPP rule, although analysts say there are few policy differences.

Electoral Commission chairman Kwadwo Afari-Gyan said Mills won 50.23 per cent in the run-off against 49.77 per cent for Akufo-Addo.

Neither candidate won a majority in the December 7th poll and run-off voting in all but one of 230 constituencies last Sunday was inconclusive, so the election was decided by Friday's voting in the rural constituency of Tain.