Tage Erlander, 84, a former prime minister who presided over Sweden's postwar economic growth and the development of one of the world's most advanced welfare states, died Friday at Huddinge Hospital near here. He was hospitalized June 9 because of pneumonia and cardiac insufficiency, doctors said.

Mr. Erlander, a Social Democrat, was named prime minister in 1946 and held office through 1969, longer than any other premier here. His governments enjoyed wide support across party lines -- something that eluded Olof Palme, his longtime personal secretary and eventual successor.

He pushed through major reforms in education, and made improvements in working hours and vacations, welfare, health insurance and pensions. The measures gave Sweden what is often described as the most elaborate social welfare programs in the Western world.

Critics said the reforms produced an unwieldy system with little room for personal initiative. Mr. Erlander defended his policies in retirement interviews by saying the public sector had yet to meet all the needs of the Swedish people.

"Socialism is to me an ideal state of society, characterized by righteousness, equality and solidarity," he said. "Those problems which are too great for the private citizen must be solved in cooperation. We must, unhampered by dogmatic ideas of the size of the public sector of society, make sure we can meet the needs of the citizens."

In foreign affairs, Mr. Erlander pursued Sweden's traditional neutrality. He was forced to back down from his support for Swedish nuclear weapons in the 1950s and he incurred the wrath of left-wing activists opposed to U.S. involvement in Vietnam after he condemned protesters as "scoundrels." He was an active supporter of the United Nations.

Mr. Erlander won a resounding victory in the 1968 elections, when the Social Democrats won more than 50 percent of the vote. He retired in 1969 to make way for Palme, who ruled until an electoral defeat in 1976. The Social Democrats regained a majority in 1982.

Tage Erlander was born on June 13, 1901, in Ransater, western Sweden, where his father was a teacher and organist. He joined the Social Democratic Party during the 1920s as a student at Lund University, where he worked as editor of a major encyclopedia from 1928 to 1938.

His rise through party ranks was swift. Elected to the Lund City Council in 1930, he became a member of parliament in 1932, undersecretary for social affairs in 1938, and minister-without-portfolio in the cabinet six years later. He began his premiership after the death in 1946 of Per Albin Hansson.

Mr. Erlander's casual style helped make him the most popular of political leaders.

He was often photographed in pajamas on the porch of his home and he was notably relaxed with visiting dignitaries. He took Soviet leader Nikita S. Khruschev and Sen. Hubert H. Humphrey on rowing trips at his country residence.

He spent his retirement lecturing and writing his memoirs, which became a best seller.

Party congresses regularly gave him standing ovations and he continued to campaign for the party at election time.

He and his wife, Aina, a chemistry and mathematics professor, had two sons.