Adama Barrow, the man who won The Gambia's disputed election, has been sworn in as president.
He took the oath at the country's embassy in neighbouring Senegal.
He has been recognised internationally. But Gambian strongman Yahya Jammeh has refused to step down and his term in office has been extended by parliament.
West African leaders have tried to persuade Mr Jammeh to admit losing the election on 1 December. They have threatened to remove him by force.
Mr Jammeh lost the poll, according to the country's electoral commission. But he wants the results annulled citing errors in the electoral process.
Mr Barrow took oath at the Gambian embassy in Dakar.
Western ambassadors to Senegal attended the ceremony, while hundreds of Gambian expatriates gathered outside the compound.
West African military forces, stationed at the border, say they are ready to enforce a transfer of power in The Gambia, a popular beach destination among European holidaymakers.
UN Security Council backing for intervention is being sought by Senegal and the regional bloc Ecowas, but some diplomats said if Mr Barrow, 51, requested help after his inauguration such approval would not be needed.
Meanwhile, Mr Jammeh's term in office has been extended by a two-third majority in parliament, and some experts say he still has a legitimate claimed to be called the country's president.
Have your say
More World Headlines
- South Africa minister resigns weeks after sex video scandal
- Brexit: Theresa May to face cabinet showdown over deal
- Trump finally appoints new ambassador to Saudi Arabia
- Over 300,000 Congolese refugees forced to leave Angola
- 10 students die in suspected arson at Ugandan boarding school
- CNN sues Trump, top White House aides for barring Acosta
- California wildfires: Death toll reaches grim milestone
- Final school hostages freed in Cameroon
- Raging Californian fires kill 25
- Study finds 'huge' fall in FGM rates among African girls
- Trump, Macron may clash on European defense in Paris talks
- Death toll from Somalia hotel attack rises to 39
- Michelle Obama: I'll never forgive Trump for putting my family in danger
- 'Don't go back to school' kidnappers told freed Cameroonian students
- US proposes rule banning asylum for illegal migrants