The nation's civilian unemployment rate fell from 6.3 to 6.1 percent last month, the lowest level in 7 1/2 years, but much of the decline may have been the result of unusual seasonal factors, the Labor Department reported yesterday.

The number of civilians employed fell by 190,000 to 112.3 million, but the size of the labor force dropped by an even larger 484,000 people. The number of unemployed people looking for work fell by 285,000, to 7.3 million.

The department said that more than two-thirds of the improvement in unemployment was accounted for by teen-agers, whose jobless rate fell almost 2 percentage points to 15.9 percent. However, the nationwide survey on which the figures are based was taken unusually early in June, when many teen-agers may still have been in school.

As a result, the seasonal adjustment factors that are used to offset the normal surge of students into the work force each June may have been too large, producing an illusive decline in unemployment. "It could be a bit of a statistical aberration," Labor Secretary William E. Brock said of the drop in the rate.

The rate has fallen a percentage point in the past 12 months. Since June 1986, the number of people unemployed has dropped by 1.1 million, and employment has gone up by 2.6 million. The last time the unemployment rate was as low was in December 1979, when it was 6 percent.

"The economy continues to grow and create jobs," White House spokesman Marlin Fitzwater said. "We continue to have low inflation, the leading indicators predict a positive growth trend and the foreign trade deficit is diminishing. These are very optimistic signs for continued prosperity."

However, some analysts have begun to worry that continued declines in unemployment could produce sufficiently tight labor markets to start to push wages up rapidly, adding to growing inflationary pressure in the economy. Economists cannot pinpoint the lowest unemployment rate that would be consistent with a stable rate of change in wages, but most believe it is around 5 1/2 percent. Economists agree it changes as the composition of the labor force changes.

Charles L. Schultze, a former chairman of the Council of Economic Advisers now at the Brookings Institution, said changes in the labor force probably have lowered the critical rate from above 6 percent to somewhere between 5 and 6 percent.

However, Schultze said that the Labor Department's employment cost index has been rising at a steadily lower rate even as the unemployment rate has declined. The latest report for that index, covering the three months ended in March, showed it had risen 3.1 percent from March 1986, he said. In the year ended the previous March the index rose 3.8 percent.

If the cost of wages and fringe benefits is rising about 3 percent a year, offset by productivity growth of about 1 percent, "then long-term growth in labor costs is about 2 percent," Schultze said, adding, "That ain't bad. For the moment, an economy with no boom at all is generating falling unemployment and falling rate of wage increase."

But Schultze is not sure what the higher rate of inflation expected for 1987 is going to do to wage demands. With higher oil prices and rising costs for imported goods -- as a result of a declining value for the U.S. dollar on foreign exchange markets -- the consumer price index will be up between 4 percent and 5 percent this year, compared with only 1.1 percent last year. "If that is the key to wages, then whether unemployment is at 6 or 6 1/2 percent may not matter" in terms of what workers may demand, he said.

"The question at the moment is not whether we are pushing the economy too hard, but the extent to which the blip in inflation raises the rate of wage increase. We probably can't stick with the 3 percent... . I am perfectly willing to say that you can go below 6 percent into the five's {in unemployment} without by itself causing problems, but it is a bad time to do it," he said.

Allen Sinai, chief economist of Shearson Lehman Bros., said the unemployment numbers are now low enough to carry a "hint of more inflation down the road coming out of a tighter job market."

Sinai said the point where the labor market becomes so tight as to ignite wage-push inflation is probably closer to 5 percent than 6 percent.

Economist David Wyss of Data Resources Inc., an economic consulting firm, agreed that the number was closer to 5 percent. "We believe we have a little room yet," Wyss said. "The hourly earnings figures surely suggest that we don't have any inflation problem in the near future."

Even though the unemployment rate has fallen by a full percentage point in the past year, the hourly earnings index -- a separate and narrower measure than the one Schultze mentioned -- has risen at a 2.3 percent annual rate over the past four months, up only slightly from last year's historically low pace of just under 2 percent.

Another part of yesterday's report, the monthly survey of industry payrolls, showed that in June, for the second month in a row, there was only a small increase in employment at business establishments. Total nonfarm employment rose by 116,000, to 101.8 million. Only 14,000 of that gain came in goods-producing industries, and manufacturing employment was virtually unchanged.

Even though employment in service-producing industries continued to grow faster than in the goods-producing sector, employment gains in the service area in both May and June were only about 100,000, much less than the usual monthly increase in recent years.

"Part of that may be the teen-agers, who normally get absorbed into the services side of the economy," said Sinai. "But we also may well have reached a saturation in services and we could be coming into a period where there is consolidation to enhance the weak productivity there rather than hiring more people."