King Sobhuza II, ruler of the tiny African state of Swaziland and the world's longest reigning monarch, died yesterday at the age of 83, Radio Swaziland reported today.

His death, after 61 years of rule, could precipitate a succession crisis in this small country situated between white-ruled South Africa and the black Marxist state of Mozambique. The succession process could lead to a power struggle among members of the royal family.

The choice among family members is wide. Sobhuza has more than 100 wives, about 600 children and untold numbers of grandchildren and great-grandchildren. It is said that more than 20 percent of the citizens of the nation of 550,000 bear the ruling family's name.

Sobhuza, called the "Lion of Swaziland," generally kept on good terms with both the segregationist government of South Africa to his west and the revolutionary government of Mozambique's President Samora Machel to the east.

But Swaziland has come under increasing pressure because of the tension between its two neighbors. Black nationalist guerrillas based in Mozambique and fighting against South Africa have used Swaziland as a path into the white-ruled country, and blacks fleeing from South Africa have come to Swaziland.

Sobhuza's death comes as South Africa has been preparing to cede territory to Swaziland that Sobhuza claimed historically belonged to the kingdom. The move is opposed by Zulu leaders in South Africa, who have warned that it could lead to tribal clashes.