Powerful forces are reshaping the American labor scene. They can't be avoided - only met head on. Companies and unions that don't realize that won't survive into the 21st century.

Arnold H. Packer, senior fellow of the Hudson Institute think tank in Indianapolis, has spent several years thinking forward to the work force of the year 2000 and beyond. At a recent labor-management conference at Northern Kentucky University, he listed three megatrends that will have a major impact:- International competition. This is perhaps the most obvious challenge.

- Demogaphics. "Those who are hiring young people are finding there aren't so many to hire," Packer said. By the year 2050, there will be fewer than one person under 20 for every person over 60. That's compared to a 7-1 ratio in 1900.

- Technological change. Productivity in manufacturing grew faster between 1970 and 1985 than in the previous 15 years. In services, however, productivity didn't improve at all. Packer expects to see gains in both in the decades ahead.

Greater productivity enables more work to be done by fewer people. That will be necessary when the baby boomers begin retiring in 20 years.

LTV Steel Corp., in a joint venture with a Japanese company, operates a new $120 million steel mill in Cleveland with just 79 employees. It's a high-tech operation requiring more brain work than back work. Everybody who works on the line is trained in Japan.

"If we're going to compete in our world marketplace, it's absolutely essential that we reshape the workplace," A. Cole Train, LTV's vice president-industrial relations, told the NKU conference.

Organized labor's survival in this new environment "will depend on its ability to change," Packer said. Unions will have to organize smaller at companies and provide new services to members.

Another conference speaker, Richard Danjin of the General Motors Corp. department of the United Auto Workers, accepted the challenge.

"We are interested in expanding market share," he said. "There are too many companies today that make money not by selling products but by bookkeeping. The only way any company can create jobs is by selling products."