NAIROBI - Senegal announced that its troops entered neighboring Gambia on Thursday to force its longtime ruler, Yahya Jammeh, to step down, part of a bold West African regional effort to defend a democratic election won by the opposition.

The operation was announced after the successor chosen by voters last month, Adama Barrow, took the oath of office from exile in the Senegalese capital.

"We have entered Gambia," Senegalese army Col. Abdou Ndiaye wrote in a text message to the Reuters news agency. There were few other immediate details of the reported incursion. Videos of camouflaged Senegalese tanks and Humvees driving toward the Gambian border circulated on social media.

By evening, the troops had not yet appeared in Banjul, the Gambian capital, according to residents, but news of their deployment had spread. Gambians poured into the streets in support of the Senegalese forces and Jammeh's ouster.

"We welcome them [Senegalese soldiers] because they will bring a president who will restore democracy here," said Modou Secka, who was part of a jubilant crowd in Banjul. He wore a shirt with the slogan that Gambians have chanted since last month's election: "Gambia has decided."

Another video posted on Twitter appeared to show Gambia's army chief, Ousman Badjie, celebrating with a crowd. Gambian soldiers were visible in the city, witnesses said, but they did not obstruct the demonstrations.

The Senegalese operation - conducted with the support of nations across West Africa - is a rare instance of an African regional military coalition responding with force to a leader's refusal to step down after an election. In recent years, many African heads of state have changed their countries' constitutions or rigged elections to remain in power, with limited opposition.

"That a regional bloc is willing to go beyond mere rhetoric, and defend the will and democratic aspirations of an entire people, speaks volumes and will undoubtedly resonate well beyond the Gambia," said Jeffrey Smith, founding director of Washington-based Vanguard Africa, a nonprofit organization that has worked closely with the Gambian opposition.

Gambia, a tiny country known for its large number of residents fleeing to Europe and a coastline that draws many thousands of British sunbathers, has been in a political crisis for weeks. Jammeh, 51, who has ruled the country for 23 years, suffered a shocking loss in the presidential election on Dec. 1. He initially conceded defeat but then quickly changed his mind and refused to step down.

Barrow, also 51, was inaugurated in a small ceremony Thursday afternoon at the Gambian Embassy in Senegal's capital, Dakar.

"My right as a winner to be sworn in is constitutionally guaranteed," he said at the event.

After the inauguration, the U.N. Security Council approved a resolution giving its "full support" to efforts by the 16-nation Economic Community of West African States (ECOWAS) to ensure that Barrow takes office, and urging Jammeh to step down. But it stressed that political methods should be used before a military operation.

U.S. State Department spokesman John Kirby said Thursday that the U.S. government supports the regional military action backing Barrow.

West African nations have tried for weeks to persuade Jammeh to step down, sending a stream of leaders to appeal to him in private meetings.

The military action reflects the region's dissatisfaction with what leaders view as unhinged leadership by Jammeh. It was also aimed at forestalling "hostilities or breakdown of law and order that may result from the current political impasse in Gambia," the Nigerian government, a member of ECOWAS, said in a statement. Nigeria and Ghana have also pledged military forces to the effort, but it was not clear whether any of them had joined the Senegalese forces in launching cross-border operations.

Jammeh's term officially expired at midnight Wednesday. Earlier that day, troops from Senegal moved to the border with Gambia.

Jammeh, a former army officer who took power in a 1994 coup, has increasingly become an international pariah. He is known for making bizarre claims, such as touting his ability to cure AIDS with local herbs. In Gambia, Jammeh's many critics say he helped enrich a small circle of politicians while doing little for the rest of the impoverished country, leading to an exodus of Gambians to North Africa and Europe.

Jammeh also vowed to slit the throats of gay men and ordered security forces to round up hundreds of people accused of witchcraft. Last year, he said Gambia would leave the International Criminal Court, which his administration mocked as the "International Caucasian Court."

Foreign diplomats had suggested in recent days that Jammeh could be offered asylum in Morocco or Nigeria in exchange for handing power to Barrow.

But on Thursday, Jammeh apparently remained in Banjul at the presidential palace. He made no public statements in the hours before Barrow's inauguration.

In recent days, thousands more Gambians have fled the country. Among them were some of Jammeh's former cabinet members, who severed ties with him after he refused to concede the December election. The country's ambassador in Washington, Sheikh Omar Faye, said last month that Jammeh "has created a serious post-election crisis and put the Gambia on a dangerous path."

Hundreds of foreign tourists, who flock to Gambia's hotel-dotted coastline, were evacuated this week.

Barrow has remained in Senegal while regional leaders tried to persuade Jammeh to leave. Barrow is a former real estate agent with little political experience - he was once a security guard at a London department store - but many Gambians see him as the symbol of a fresh start for the country. Some of his supporters suggested that they would be willing to fight Jammeh's forces if necessary.

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Saikou Jammeh in Banjul and William Branigin in Washington contributed to this report.