Gambian President Yahya Jammeh has rejected the result of the election which he lost earlier this month.
In an announcement on state TV, he said the electoral process had suffered "unacceptable abnormalities" and he called for a fresh vote.
Mr Jammeh, who came to power in a coup in 1994, had suffered a surprise defeat to Adama Barrow, who won more than 45% of the vote.
Mr Jammeh initially accepted the result, surprising his critics.
"After a thorough investigation, I have decided to reject the outcome of the recent election," Mr Jammeh said on Friday.
"I lament serious and unacceptable abnormalities which have reportedly transpired during the electoral process.
"I recommend fresh and transparent elections which will be officiated by a god-fearing and independent electoral commission."
The Gambia is the smallest country on mainland Africa, with a population of fewer than two million. Tourism has become its fastest-growing sector.