Consumer prices rose 0.2 percent in September as falling mortgage interest rates and gasoline costs partially offset increases in food, medical care and used cars, the Labor Department reported yesterday.

The increase left the consumer price index up only 5 percent over the last 12 months, the smallest such rise since the year ended in December, 1976. The 5 percent increase is down significantly from last year's 8.9 percent rise and the 12.4 percent jump in 1980.

For the last three months, from June to September, the CPI rose at a seasonally adjusted annual rate of 4.2 percent.

But just as inflation has come down, so have the size of wage increases and weekly earnings as recession has cut into both. The Labor Department said yesterday that average weekly earnings of production and non-supervisory workers fell 0.1 percent in September, which, combined with the 0.2 percent rise in the CPI, left real weekly earnings down 0.3 percent.

Over the last 12 months, real weekly earnings have fallen 0.9 percent, from $168.05 to $166.87, the department said.

With congressional elections only a week away, the CPI figures drew immediate political comment. President Reagan, campaigning in Raleigh, N.C., for Republican House candidates, told a rally that, "as of today," inflation is "back to where it was in January of 1977, when Gerald Ford left office. We are clearing away the economic wreckage dumped in our laps."

Countering for the Democrats, House Speaker Thomas P. (Tip) O'Neill replied, "Today's consumer price news is nothing to brag about. It is the direct result of the worst recession since the 1930s. Of course prices are not rising as fast as they were; no one is buying anything."

Jerry Jasinowski, chief economist for the National Association of Manufacturers, said of the September figures, "The continued decline in the inflation rate is due primarily to the recession. High unemployment has forced wage increases down, and falling demand has squeezed profit margins."

Jasinowski predicted, "The inflation rate is likely to average about 5 percent this year, and should remain in this range for the next several years."

In September, the CPI rose to a level of 293.3, which means that it took $293.30 to buy the same marketbasket of goods it took $100 to buy in 1967.

Last month grocery store food prices rose 0.5 percent, reversing a 0.6 percent decline the month before. Egg and pork prices were up sharply -- pork prices have risen 20 percent since the beginning of 1982 -- while fruit and vegetable costs fell.

Medical care costs rose 0.9 percent, just about the same rate as in every other month this year, and are now up 11.4 percent in the last year. Used car prices continued to increase sharply, up 1.2 percent for the month and 11.7 percent for the year.

Offsetting those increases was a 0.7 percent drop in homeownership costs, primarily as a result of a 1.4 percent drop in mortgage interest rates.

Gasoline prices went down 0.1 percent for the second month in a row and are now 7.8 percent below their peak of March 1981.

Meanwhile, the Treasury Department released official federal budget figures for fiscal 1982, which ended Sept. 30, showing the largest deficit ever, $110.7 billion. The deficit was far larger than that of 1981, $57.9 billion, and the previous high set by the Ford administration in 1976, $66.4 billion.

Federal fiscal year receipts were $617.76 billion, the department said, $4.3 billion less than last estimated in July. Federal outlays were $728.4 billion, $2.6 billion less than last estimated.

Meanwhile, The F.W. Dodge Corp. of McGraw-Hill Inc. said the value of new construction contracts awarded in September advanced a seasonally adjusted 4 percent to $14.2 billion, extending the generally upward trend of the past six months. Dodge economist George A. Christie called this "better-than-expected."