The unemployment rate among District of Columbia residents dropped nearly 1 percent in Septmember despite the fact that fewer people were working, the D.C. Labor Department reported yesterday.

The jobless rate, adjusted for seasonal variations, was 6.6 percent for city dwellers in September, a decline from 7.4 percent in August and from 8 percent in September 1978.

The local trend magnified what happened nationally in September, when the jobless rate unexpectedly dropped to 5.8 percent compared with 6 percent in August.

Locally, however, the trend was potentially ominous. There were 8,100 fewer D.C. residents working in September 1979 than a year previously. The jobless rate -- the proportion of local people without jobs -- went down because there were 13,400 fewer people in the labor force. The labor force is the total number of people holding or seeking jobs.

In september 1979, there were 314,000 in the city's labor force and 293,600 working. In September 1978 there were 327,400 in the labor force and 301,700 working.

For the metropolitan area as a whole, counting suburbanites as well as D.C. residents, the adjusted jobless rate for September 1979 was 4.4 percent, compared with 4.5 percent in August and 4.7 percent in September 1978.

Yesterday's report did not indicate the ages, race or sex of those are unemployed.