Gambia's longtime leader Yahya Jammeh said on Saturday he would step down from power in a statement made in the face of pressure from West African armies that invaded his country to remove him after he refused to concede an election defeat to President Adama Barrow.

"I have decided today in good conscience to relinquish the mantle of leadership of this great nation," he said on state television, wearing a white robe and looking tired.

Jammeh lost an election to Barrow in December and initially conceded defeat, but changed his mind saying the vote had been unfair.

Jammeh said his decision was taken in the national interest after prayer and said he was proud to have served the Gambian people, imploring them to work together as one nation.

His statement will be welcomed by democracy advocates across Africa and brings to an end a reign that began in 1994 when he seized power in a coup. His government established a reputation for torturing and killing perceived opponents to stifle dissent.

Barrow was sworn in on Thursday in neighbouring Senegal and late that night West African forces from regional bloc ECOWAS crossed the border and were poised to strike the capital to enforce the new president's mandate and resolve the impasse.

Guinea's President Alpha Conde and Mauritania's President Mohamed Ould Abdel Aziz traveled to the capital Banjul on Friday to allow Jammeh one last chance to cede power peacefully.