Senegalese troops have entered The Gambia in support of Adama Barrow, who was sworn in as president on Thursday after winning last month’s election.
Mr Barrow took the oath of office at the Gambian embassy in Senegal’s capital, Dakar, and his legitimacy has been recognised internationally.
But the strongman he defeated, Yahya Jammeh, is refusing to quit and is supported by Gambia’s parliament.
West African leaders have threatened to remove him by force if necessary.
The 15-member UN Security Council has given them its backing, while stressing that a political solution should be attempted first.
Nigeria deployed reconnaissance aircraft over The Gambia on Thursday as part of the mission, warning that it was ready to strike militarily.
The tiny country, a popular destination for European holidaymakers attracted by its beaches, has been clouded by uncertainty for weeks.
Thousands of Gambians have sought refuge in Senegal while tourists broke off their holidays to return home.
People have been following the inauguration of Adama Barrow live on Senegalese TV, which many receive here in The Gambia.
Few people have come out to celebrate, timidly chanting the name of Mr Barrow or waving at the cars driving by. Tension is still running high, as people are very much aware that the political crisis is not over.
Banjul feels like a ghost town. Even the usually busy thoroughfares of Serekunda, on the outskirt of the capital, are deserted. Many say the military remain – like Yayha Jammeh – unpredictable.
But in a sign that parts of the security forces may switch sides, I have met five police officers standing outside their station, relaxed and visibly happy. I asked how things were going, and one of them replied with a smile “everything is alright, change is good”.
BBC