Senegal's military has entered Gambia as part of regional efforts to support its new President Adama Barrow and remove longtime ruler Yahya Jammeh, who has refused to step down after last month's election.
Barrow took the oath of office as Gambia's president on Thursday at its embassy in neighbouring Senegal, calling for international support from West Africa's ECOWAS bloc, the African Union and the United Nations.
"We have entered Gambia," Senegal's army spokesman Colonel Abdou Ndiaye told Reuters.
Nigeria, which pre-positioned war planes and helicopters in Dakar, said in a statement that it had also deployed military assets "to protect the people of the Gambia and maintain sub-regional peace and security."
It was not immediately clear, however, if they too had crossed the border.
Ghana has also pledged troops.
"This is a day no Gambian will ever forget," Mr Barrow said after taking the oath, which was administered by the president of Gambia's bar association.
"Our national flag will now fly high among the most democratic nations of the world.
"I hereby make an explicit appeal to ECOWAS, the [African Union] and the UN... to support the government and people of the Gambia in enforcing their will, restoring their sovereignty and constitutional legitimacy," he said.
The UN Security Council on Thursday backed ECOWAS's efforts to ensure Mr Barrow assumes power, and the United States said it supported Senegal's intervention.
ECOWAS has been attempting to persuade Mr Jammeh to quit for weeks, and has failed to do so, despite his increasing political isolation and last ditch efforts to reason with him overnight.
Mr Jammeh, in power since a 1994 coup and whose mandate ended overnight, initially conceded defeat to Mr Barrow following a December 1 election before back-tracking, saying the vote was flawed.
Hundreds of Gambians celebrated in the streets, cautiously at first, and then gradually in larger numbers as they realised the security forces looking on were not going to open fire.
Cars whizzed up and down the highway lined with iron-roofed shops in the pro-Barrow Serrekunda district of Banjul, with horns honking and people hanging out of the windows.
"The dictator is out," shouted pharmacist Lamine Jao, 30, as others cheered and whistled in agreement. "It's just a question of time. We'll soon flush him out. Believe me," he said.
During the brief inauguration speech, Mr Barrow asserted his new role as commander and chief of Gambia's armed services, ordering soldiers to stay calm and remain in their barracks. Those who did not would be considered rebels, he said.
ECOWAS and the African Union have said they will recognise Mr Barrow from Thursday and nations including the United Kingdom and France were quick to congratulate Mr Barrow.