Serious crime in the District of Columbia jumped 10.8 percent in 1979 over 1978, the biggest annual increase in a decade -- and a grim and concrete signal, senior police officials say, that the city is experiencing a new period of hard-core, hard-to-eradicate street crime.

Year-end figures released by the D.C. Police Department yesterday showed that 56,430 serious offenses -- murders, rapes, robberies, aggravated assaults, burglaries, larcenies and auto thefts -- were reported in 1979, compared to 50,950 in 1978.

That increase is the greatest single leap since the tumultuous years following the King riots of 1968 when Richard Nixon, campaigning for the presidency, branded Washington the "crime capital of the nation." The crime rate here leaped 20 percent from 1969 to 1970.

But officials were quick to point out yesterday that, even with the dramatic increase in reported crime in 1979 over 1978, the totals are still substantially below the peak year of 1970, when more than 90,000 major crimes were reported.

After 1970, reported crime decreased rapidly for three years to the 50,000-a-year range, and has been bouncing up and down erratically ever since.

Put another way, the average number of serious crimes reported in the city each day fell from a peak of 265 in November 1969, to a low of about 120 a day during various months in the middle and late 1970s. That daily average is now back up to at least 165.

The sudden 11 percent jump in 1979, following an almost insignificant increase of less than 1 percent the previous year, however, now has some police officials worried. Officials from a variety of branches and levels of the department gave these widely varying factors as possibly contributing to the sudden surge in crime:

Worsening economic conditions. Unemployment is up, especially among teen-agers who commit a large portion of the city's crimes. Inflation is forcing some people, especially the elderly on fixed incomes, to steal more to make ends meet.

Increased numbers of heroin addicts. More addicts are committing more crimes -- mostly burglaries and larcenies -- to support their expensive habits. c

Lower morale in the police department. Rank-and-file officers, lacking the incentives of promotions and higher pay, are not policing as aggresively as in previous years.This gives street hustlers, boosters and other assorted petty criminals a freer hand.

The influx of middle-class people to the city. A marked "return" of professional and white-collar residents, especially around the fringes of high-crime areas has resulted in a higher percentage of actual crimes being reported to police. Low-income residents, many of whom are being displaced by the new middle-class and move out of the city, have traditionally been less inclined to report crime to the police.

At a press conference where he released the 1979 crime figures yesterday, D.C. Police Chief Burtell M. Jefferson briefly mentioned only heroin and worsening economic conditions as probable factors in the crime rise.

He said he was "not particularly happy" with the figures, noting that all categories of crime showed increases except homicide.

He hinted that the situation could worsen if proposed budget cuts necessitated by a recently discovered revenue shortfall in the city require him to reduce his 3,900-member force.

"We need big visibility to keep crime down," he said. "Any cuts (in manpower) would hurt me."

He said while serious crime as a whole was up almost 11 percent, more than half of that increase was attributable to a large jump in larcenies, especially thefts from autos and office buildings.

Larceny, classified by police as a nonviolent "property" crime, is the most commonly reported serious offense and outweighs the less commonly occurring violent "personal" crimes of murder, rape, robbery and assault.

Larcenies in 1979, for example, to taled 28,819, compared to 25,744 in 1978, an increase of 3,075. By contrast, there were 6,920 reported robberies in 1979, compared to 6,333 in 1978, an increase of 587.

Also, there were 180 homicides in 1979, nine fewer than in 1978. Other figures show 402 rapes in 1979 compared to 381 in 1978 and 2,964 aggravated assaults, compared to 2,546 the previous year.

In other property crimes the figures show 13,452 reported burglaries in 1979, compared to 12,497 in 1978, and 3,606 auto thefts in 1979, compared to 3,194 in 1978.

On the causes of the upsurge in crime, several rank-and-file officers and some mid-level captains and lieutenants said privately that low morale has discouraged aggressive patrolling.

They attribute the morale problem to anticipated budget cuts that may limit promotional opportunities and force officers out of gas-guzzling scout cars and into walking beats.

Also, some say, the abolition of the traditional "efficiency rating" by supervisors of officers seeking promotion eliminates an officer's incentive to do a good job. The efficiency rating was abandoned last year, in part because some officials said it encouraged favoritism, including racial preferences by white supervisors.

Some white officers are now grumbling that the new promotion system, consisting of a written exam and a structured "oral interview" by senior officials, is effectively controlled by the black hierarchy of the department and may ironically be used by them for the same kind of racial favoritism, only in reverse. Test papers currently being prepared for a March 15 examination, they note, require candidates for promotion to specify their race. e

Police view the increased availability of heroin as a primary contributor to rising crime here. All statistical indices show heroin use is increasing: city-run treatment centers are taking in more addicts; purity levels of street-level heroin seized by police are higher than in past years; heroin over-dose deaths are up dramatically -- 34 in 1979, compared to only 7 in 1978.