A young man in his late teens stopped recently in front of the large window of McDonald's restaurant near the corner of 13th and E Streets NW., took a long drag on his short stub of a marijuana cigarette, briefly glanced around, then extinguished it with the heel of his shoe.

The young man did not seem to mind that neither those inside the restaurant nor the lunch-time crowd that passed him on the sidewalk could see him smoking the illegal substance.

In the District of Columbia, possession of small quantities of marijuana for personal use is quietly - in fact although no in law - being discriminalized because law enforcement officials do not believe that cracking down on small-time offenders is worth the effort. The officials say they respond to public opinion in not cracking down.

Although many cases involving simple possession of marijuana are dropped, a first offender brought to trial on that charge has a good chance of receiving only a fine ranging up to $100, a suspended sentence carrying a one-year probation, or both. An ounce of marijuana is equivalent to about 16 marijuana cigarettes.

While officials insist that they do uphold drug laws, the large number of possession cases dropped, the number of offenders referred to drug education programs and even the penalties give upon conviction indicate that officials consider possession of an ounce or less of marijuana a low-priority offense.

Judges, prosecuting attorneys andpolice officers in the District of Columbia have admitted openly that they consider that the effort used in bringing small possession cases to trial could be used more effectively in fighting more serious crimes.

Ninety-five per cent of those arrested for marijuana possession last year were charged because police found the drug on the offender's person at the time of arrest or during a search for other offenses, police said. A sizable majority of those cases involved motorists being stopped for traffic violations, police said.

Police said their judgements and attitudes are influenced greatly by current social attitudes that, for the most part, deem personal use of marijuana almost as acceptable as drinking a cocktail.

"I don't want someone to get a false sense of security," said Capt. Houston M. Bigelow, chief of the D.C. Police narcotics branch.

"No policeman will ignore a violation of law," he said. "But we're a part of society and when the White House talks about decriminalization and seven states already have decriminalized marijuana, it's only a natural reaction for the community and everyone else to take less than a serious look at it."

U.S. Attorney Earl J. Silbert said his attorneys who prosecute marijuana possession cases often dismiss charge against first offender "if jutime might be better served by doing so."

"Prosecutors are influence by current attitudes and by the people they serve," Silbert said.

According to figures provided by Silbert's office, 1,570 arrests were made last year for possession of one ounce or less marijuana. Only 1,144 of those cases were filed in court, and only 327 guilty verdicts were returned.

"I don't have the figures, but I would think that most of those found guilty were given fines or probation in lieu of a sentence if they didn't have a prior criminal record or if they weren't found with a considerable amount (of marijuana)," Superior Court Judge Fred L. McIntyre said.

Efforts to change drug laws to provide the type of penalties that law enforcement officials believe will better match the seriousness of the violation have met with stiff opposition from religious groups and some key congressional leaders. This has occurred despite President Carter's recommendation last week that simple possession of marijuana be decriminalized.

Two years ago, for example, an attack by the city's Committee of 100 Ministers exerted enough political pressure on Mayor Walter E. Washington and the City Council to kill a bill providing for decriminalization of marijuana.

At the same time, Rep. Charles Diggs (D-Mich.) chairman of the House District Committee, vehemently opposed any legislation that would lessen the penalties for possession. Neither Diggs nor the Committee of Ministers has softened significantly their position on marijuana possession.

Legislation now before the City Council would revamp all drug laws along much the same lines as federal and local laws, with the exception of those pertaining to marijuana.

Possession of an ounce or less is handled under District law, which assesses up to a $1,000 fine, one year in prison or both. U.S. attorneys have said privately that the maximum penalty for simple possession is "unrealistic" and is never given in the District.

The proposed District law would decriminalized possession by removing the criminal record that a violator would receive if arrested. The penalties - up to a $100 fine and a citation for the first two offenses, up to a $100 fine for the third offense and mandatory drug education classes - do not differ much from the types of punishment that courts now mete out.

Regardless of whether the Council passes the proposed legislation that includes decriminalization of marijuana, some city residents apparently intend to continue pursuing what they consider their "right" to smoke.

Three well-dressed men, each angling his body so he could look in a different direction, sat recently on a concrete park bench underneath a tree across the street from the District Building. They passes around a marijuana cigarette.

They watched an approaching reporter, unknown to them, and the man with the lighted cigarette slowly dropped the hand containing the marijuan cigarette beside on of his legs. Crumpled sandwich bags surrounded his ankles.

"We know the cops are not going to bust us for just one join (cigarette) of marijuana," one of the three told the reporter.

"If they do come, we can get rid of it in a hurry. But I don't think any of us are worried about it," another said.