The official unemployment rate among District of Columbia residents dropped in December to 7.1 percent, the lowest monthly figure in 3 1/2 years, the D.C. Department of Labor reported yesterday.

Among Maryland and Virginia suburbanites, the jobless rate for December was listed at 3.4 percent, well below the national figure of 5.9 percent announced previously.

The District rate -- which, experts agreed, masks a far higher jobless rate in poverty-plagued inner-city neighborhoods -- was the lowest since a 6.9 percent figure in May 1975. After that, unemployment among city residents rose, reaching an official peak of 11 percent in June 1977.

While employment usually rises during the Christmas shopping season, the Labor Department said December was better than expected for city residents, with 313,600 working and 23,900 jobseekers listed as unemployed. In the same month a year earlier there were 299,600 working and 28,400 unemployed.

In the suburbs, 1.2 million residents were listed as employed and 43,000 jobless in December, a figure virtually unchanged from the previous year.

The report did not contain a breakdown of the jobless figures by race, sex or age. The Labor Department said those figures for 1978 are still being compiled.

Coming two days after the Washington Urban League issued a report showing what it called "an extremely high" 24 percent without jobs last June in inner-city neighborhoods, the new Labor Department report revived an old debate over the accuracy of unemployment statistics.

John Watkins, communications director for the local Urban League, said the official figures drastically understate the jobless rate, since they do not include would-be workers who become discouraged by a repeated failure to find jobs and those who do not seek work through agencies that keep statistics.

Of the unemployed who were surveyed, Watkins said, one-third reported they were discouraged and not then looking for jobs. He urged that statistics be collected to reflect such conditions.

Last summer, for example, official figures showed that joblessness among black teen-agers in Washington during 1977 had reached 48.5 percent. The Urban League contended the actual figure was close to 70 percent.

John Gallahan, a labor economist who helped compiie the official figures, agreed that they understate the actual number of people out of work. But he said the official definition used nationally to collect jobless figures contributes to that result.

"Basically," Gallahan said, "the definition of someone unemployed is someone who did not work during the survey week for pay and who has made some positive effort to look for work in the past four weeks."

The monthly figures are based heavily upon unemployment compensation payments and contacts made by jobless people with government-sponsored employment offices, supplemented and adjusted by detailed periodic surveys made by the Census Bureau.

Watkins, the Urban League spokesman, said many inner-city residents -- especially the young ones -- never have qualified for unemployment compensation and tend to seek jobs through informal channels.

James O. Gibson, the newly appointed D.C. assistant city administrator for planning and development, said the Urban League's jobless figures are "shocking in social terms." But he speculated that an analysis would show them not too far off from the official figures if they were available on a neighborhood-by-neighborhood basis.

Gibson also said the improved employment figure for December seems to reflect the migration of lower-skilled people out of the city and an influx of better-educated jobholders to the city.

Henry Loss, metropolitan director for the Natioal Alliance of Business, a government-industry group that seeks jobs for minority youth, said the absence of reliable statistics poses a major problem.

In compiling a recent report, Loss said he found figures for "the socalled discouraged worker" in the District ranged between 4,000 and 17,000, depending upon the source.

All those interviewed yesterday agreed that the official unemployment statistics represent a useful gauge of job trends, even if the figures are not precise.

Thomas Wilkins, acting director of the Labor Department, said the December figures represent the third consecutive month in which unemployment in the city has dropped. In November, the figure was 7.9 percent. Joblessness is normally highest in the summer, when many youths seek work.

On a metropolitan-wide basis, the department said there were 70,000 more people at work in December than there were in the same month of 1977. Of the new jobs, 9,000 were in the District. At least 5,000 District residents -- and probably more -- found new jobs in the suburbs.

Among industry groups, there were about 3,000 more jobs in manufacturing, 10,000 more in construction, 13,000 in wholesale and retail trade, 4,000 in the financial field, 9,000 in government (6,000 federal) and mearly 30,000 in various service industries.