PITTSBURGH (AP) _ United Steelworkers leader Lynn R. Williams was a boy when he got his first glimpse of the labor movement, watching a sit-down by foundry workers until his father dragged him away ''because he was afraid of what we might get involved in.''

On Saturday, Williams took the oath of office in Toronto for his first full term as president of the United Steelworkers of America.

''Dad was a minister with a working-class congregation, and ... there were Christmas Eves I can remember taking food baskets to families. It sounds corny, but I thought this isn't the way things should be,'' Williams, 61, said in a recent interview in Pittsburgh, headquarters for the international union.

Williams, born in Springfield, Ontario, in 1924, recalled the strike against the Canadian foundry.

''A couple of us kids were up there watching, and my dad came along and dragged us away because he was afraid of what we might get involved in,'' he said.

Decades later, he continues to espouse the labor movement.

''If there ever was a time, just as there was 50 years ago, when working people need the voice in their future that only a strong union can provide, that time is now,'' he said in a speech prepared for the Toronto ceremony.

Williams was appointed acting president of the union in November 1983 after the death of Lloyd McBride, the union's fourth president. Williams won a bitter election in March 1984 over union Treasurer Frank McKee to finish McBride's term. He was declared president without opposition late last year.

The college-educated Canadian came to power as imports, an economic recession and the rise of cheap Third World labor was breaking the USW's tradition of fatter and fatter contracts.

Active membership in the USW has shrunk to about 700,000, half the number of the early 1970s. Fewer than 200,000 members hold jobs in the shrinking U.S. steel industry.

Eight months before Williams replaced McBride, the union had signed a national steel contract that reduced wages and benefits for the first time.

More concessions followed when union members settled a 98-day strike early this year against financially troubled Wheeling-Pittsburgh Steel Corp., and union bargainers at six other big steelmakers continue to backpedal in negotiations to replace the 1983 pact, which expires on July 31.

''Think what would have happened if we didn't have a union,'' Williams said. ''Unions have kept things from getting much, much worse.''

Williams is the first Canadian to head the USW and the first non-American to lead a major U.S. union. After nearly a decade on the international staff in Pittsburgh, he remains a Canadian citizen who believes that organized labor should follow multinational corporations across national boundaries.

''These corporations have become principal players on the world stage ... dealing with many nation states from a power base that exceeds that of the states themselves,'' he said in his speech. ''The labor movement has much to do in order to develop the ability to cope with the many manifestations of their power.''

Williams became a social worker for the YMCA and attended college on a YMCA scholarship. He became involved in a strike against the Hamilton steelmaker Stelco in 1946, and began his formal career as a Steelworkers organizer the next year.

''This wasn't do-gooder stuff, charity, handing out alms,'' he said. ''If you looked at the people, they didn't need a social worker visiting them making their decisions. They needed the opportunity to make decisions for themselves, and that's what the labor movement did.

''It was a source of pride.''