King Leopold III, 81, who was forced to abdicate his throne as king of the Belgians in 1950 for having surrendered his nation to invading German forces in 1940, died Sept. 25 at Louvain University Hospital here after a heart attack.

King Leopold was born in Brussels. He ascended to the throne on Feb. 23, 1934, becoming Belgium's fourth monarch since the nation gained independence from the Netherlands in 1830. He succeeded his father, King Albert, who died in a mountain climbing accident in southern Belgium.

In 1940, when German armies invaded Belgium, King Leopold personally led the Belgian resistance for 18 days and then capitulated, bringing harsh criticism from Belgium's French and British allies, and from his own government in exile. Relations between the king and his government deteriorated when he refused to follow them into exile, saying that as supreme army commander he wanted to stay with his soldiers.

King Leopold sat out the war a virtual prisoner in his Laeken palace on the outskirts of Brussels. Opposition to the king gained further momentum after the war when it became known he had paid a secret visit to Hitler, although King Leopold said the trip was to obtain the release of Belgian prisoners of war and assurances about Belgian independence at the war's end.

Toward the end of the war, the king was taken by the Germans first to Germany and then to Austria, where he remained until the end of hostilities.

King Leopold also was unpopular for his second marriage, a morganatic union in 1941 with the London-born Mary-Lilian Baels, the daughter of a former province governor, whom he made Princess de Rethy. The marriage took place in a civil ceremony.

At the time, the new princess was pregnant. It was later revealed that the couple had wed in a church ceremony three months earlier. In heavily Roman Catholic Belgium, a civil marriage ceremony must precede the church rite.

King Leopold's first wife, the Swedish-born Queen Astrid, whom he married at the age of 26, died in August 1935 when a sports car in which she was riding hit a tree and plunged into a Swiss lake. The king, who was at the wheel, suffered minor injuries.

Because of these controversies, King Leopold refused to return to Belgium and take up the throne until a popular referendum resulted in a vote of more than 55 percent favoring his return. In 1950, widespread riots rocked the country and three people were killed in a riot near Liege. King Leopold agreed to step down in favor of his son, Baudouin, who formally took the throne on Sept. 7, 1951, taking his oath as King of the Belgians.

Over the years, King Leopold had become fascinated by South American culture. He went on several expeditions to Venezuela, Colombia and Panama in the 1950s. He also travelled to Africa and Asia after his abdication, devoting much of his time to scientific research. The Belgian Foundation for the Fight against Leprosy was created at his initiative.

King Leopold had three children by his first marriage; King Baudouin, who married Spanish-born Fabiola de Mora y Aragon; Princess Josephine Charlotte, who is married to Grand Duke Jean of Luxembourg; and Prince Albert, who is married to Italian-born Princess Paola Ruffo di Calabria. He also is survived by three children by his second marriage, Prince Alexander, Princess Marie-Christine and Princess Maria Esmeralda. King Leopold's younger brother, Prince Charles, who acted as regent of Belgium from the liberation until Leopold returned in 1950, died last June at age 79.