Ghana's main opposition party has filed a petition at the Supreme Court to challenge President John Mahama's victory in this month's election.

The New Patriotic Party (NPP) rejected the results of the 7 December poll, alleging fraud.

The election commission said Mr Mahama had secured 50.7% of votes, enough to avoid a run-off against NPP candidate Nana Akufo-Addo with 47.7% of the vote

International election observers described the poll as free and fair.