The nation's civilian unemployment rate rose slightly to 7.1 percent last month as a surge of more than half a million persons into the labor force more than offset a strong gain of 370,000 jobs, the Labor Department reported yesterday.

However, the number of manufacturing jobs dropped by 110,000, the largest one-month loss since the end of the 1981-82 recession. Manufacturing employment has declined by about 340,000 since January, due primarily to a large rise in the volume of goods imported from other countries.

The volatile unemployment rate for blacks rose to 15.3 percent after dropping a full percentage point to 14 percent the previous month. The rate for whites fell from 6.2 percent to 6.1 percent.

The September rate compared with the rate of 7 percent in August. A separate measure of the unemployment rate that includes members of the armed forces in the labor force rose last month from 6.9 percent to 7 percent.

The continued relatively high level of unemployment, and the need to meet the threat of imports, are holding wage increases to their lowest level in many years.

The department said its index of hourly earnings paid to production and nonsupervisory workers on private nonfarm payrolls was essentially unchanged both between August and September and between the second and third quarters. Over the last 12 months, the index is up only 2.4 percent, the smallest rise recorded since the index was started in 1964. The previous low was 2.7 percent in January and July, a department analyst said.

Robert Ortner, the Commerce Department's chief economist, termed the employment report, the first major indicator of economic growth for September, as "mediocre at best." Ortner said that the overall gains shown are consistent with an increase in third-quarter gross national product and only "a little better" than the 2.8 percent annual rate given last month in his department's so-called "flash" report. That figure was after adjustment for inflation.

Other analysts noted that even though the length of the average factory workweek rose by one-tenth of an hour, with the decline in jobs the total number of hours worked in manufacturing fell. The same was true in mining. That suggests that there was little, if any, increase in industrial production in September, the analysts said.

"The economy is still in low gear," said Norman Robertson, chief economist of Mellon Bank in Pittsburgh, after looking at the report. "We're on a slow, unsteady growth track."

The continued economic problems faced in some parts of the country were underscored by September unemployment rates of 10.9 percent in Michigan, and 9.4 percent in Ohio and Illinois. Massachusetts, on the other hand, had the lowest rate, 3.8 percent, among the 11 largest states covered in yesterday's report.

Janet L. Norwood, commissioner of labor statistics, told Congress' Joint Economic Committee that last month's increase in the unemployment rate from August's 7 percent level still left the figure below the February-July plateau of 7.3 percent.

"The civilian labor force broke out of the no-growth pattern which existed from March to August and increased by half a million in September, with most of the gain about evenly split between adult men and women," Norwood said. In the past 12 months, the civilian labor force has increased by about 2 million workers, with about four-fifths of the rise occurring among adult women.

"At 98.1 million, the overall number of nonfarm payroll jobs was up very slightly from the August level," she continued. "Gains took place throughout the service-producing sector and in construction. In contrast, manufacturing employment declined by 110,000, with the largest losses in machinery, electrical equipment, and motor vehicles. Part of the decline in automobile employment resulted from strike activity and part from the changed patterns of retooling for new cars . . . "

Norwood said that since January the number of jobs in services has gone up by 770,000, in retail trade by 450,000 and in construction by 190,000. "In manufacturing, however, there has been a loss of 340,000 jobs, with nearly half of the drop occurring in machinery and electrical equipment."

Labor Secretary William E. Brock issued a statement that stressed the positive aspects of the employment report, mentioning neither the rise in the unemployment rate nor the drop in manufacturing jobs: "Almost 109.3 million Americans were working in September -- another record high. And 8.5 million Americans have been added to the payrolls in the last 34 months -- an average of 250,000 a month . . . . The length of the manufacturing work week in September grew to 40.7 hours -- the longest in more than a year. Meanwhile, our service and construction industries continued to register healthy employment increases."

The department said the unemployment rate for adult men was unchanged last month at 6 percent; the rate for adult women rose from 6.7 to 6.8 percent, and the rate for teen-agers, which had dropped from 19.5 percent in July to 17.3 percent in August, went back up to 17.8 percent.