President Yahya Jammeh has declared the Gambia "an Islamic state", but stressed that the rights of the Christian minority in the small west African country would be respected and that women would not be held to a dress code.
The announcement came as the president addressed supporters in the coastal town of Brufut on Thursday (local time), and the comments were later broadcast on state television and repeated on his website.
"Gambia's destiny is in the hands of the Almighty Allah. As from today, Gambia is an Islamic state. We will be an Islamic state that will respect the rights of the citizens," he was quoted as saying on the presidential website.
In television footage of the address on GRTV, seen by AFP on Saturday, the president did not go into detail about what the change would mean for the country, but he reassured Christians and followers of other faiths they would be able to worship freely.
"Christians will be given their due respect. The way of celebrating Christmas will continue," he said, adding that no one had the right to interfere with others' "way of life".
He also warned against trying to impose a dress code on women.
"I have not appointed anyone as an Islamic policeman. The way women dress is not your business," he said.
An impoverished former British colony nestled within Senegal, and famed for its white-sand beaches, the Gambia has a population of nearly two million, 90 percent of whom are Muslim.
Of the remainder, eight percent are Christian and two percent are defined as having indigenous beliefs.
Jammeh, 50, a military officer and former wrestler from a rural background, has ruled the country with an iron fist since he seized power in a coup in 1994.
The main opposition party on Saturday poured scorn on the president's unexpected proclamation of an Islamic republic, saying it had no legal basis.
Earlier this year, Human Rights Watch branded Jammeh's regime one of the most repressive in the world, blaming paramilitaries and secret police for torture, disappearances and extrajudicial killings.
In 2013, Jammeh withdrew his country from the Commonwealth, saying it represented "an extension of colonialism".