The D.C. City Council yesterday gave its final - but narrow - approval to a bill that would decriminalize penaltties for the possession of small amounts of marijuana. At the same time, the Council strengthened the city's laws affecting trafficking in harder drugs, such as heroin.

The Council's 7-to-6 approval of the bill, with no change in votes from the preliminary approval given Oct. 9, set the stage for a possible veto of the measure by Mayor Walter E. Washington, who must act within about two weeks.

The city's black clergymen, led by the Committee of 100 Ministers, has lobbied strongly against passage of the marijuana descriminalization provisions of the bill. That group was joined recently by the U.S. Labor Party and the Blackman's Development Center Inc., a community organization involved in various drug-treatment programs.

At the same time, the D.C. Young Democrats and the National Organization for Repeal of Marijuana Laws, have urged passage of the measure. The latter group has contributed to the campaigns of some Council members.

A spokesman for the mayor refused to say yesterday how Washington would act. Several persons in city hall have speculated privately, however, that because of the mayor's close relationship with the ministers in the past and the current political climate, a mayoral veto appears likely.

The mayor is considering running for election to a second term next year. Only last month, he scored some political points, in the view of some observers, with his surprising veto of legislation that would have permitted immediate rent increases of from 2 to 10 per cent. The City Council, in effect, sustained the mayor's veto.

The legislation approved yesterday authorizes issuance of a citation similar to a parking ticket for possession of one ounce or less of marijuana. Each of the first three citations issued could be accompanied by a fine fo up to $100.Beginning with a fourth offense, possession would be punishable by a fine of up to $1,000 and a year in jail.

The new bill would provide stiffer sentences for repeat violators of dru-trafficking laws. Under present law, repeat offenses carry the same penalty as first offenses. The new law would double the penalties for second offenses.

The new legislation, which basically brings city drug laws into conformance with federal laws, will not give the city authority to prosecute felony drug violations. Those prosecutions will continue to be done by the U.S. Attorney's Office.

But the new law does, for the first time, allow such cases to be tried in D.C. Superior Court (they are now tried exclusively in federal court) and backers of the bill hope that will increase the opportunities for felony prosecutions.

In addition, some city officials hope that in the future, the city will obtain authority from Congress for the corporation counsel to undertake criminal prosecutions.

Although none of the Council's 13 members changed his or her voted from Oct. 13, yesterday's final action was preceded by more than an hour of emotional statements by Council members, including some moments where various members pounded their desks to emphasize their positions.

Among the arguments advanced in favor of passage of the bill was that, under present law, marijuana law violators are given a criminal record that simply indicates violation of the Uniform Narcotics Act. That could mean anything from heroin possession to marijuana use. As a result, it is said, some city residents fare poorly in job applications because use of stronger drugs is implied.

The new law also is seen, in some respects, to be a formality as far as marijuana use is concerned because city police rarely enforce marijuana use provisions. Moreover, Councilman David A. Clarke (D-one) argued that the bill is a comprehensive drug package, which also increase penalties for sale of heroin and PCP, and allows the mayor, by his own action, to increase the penalties for use of marijuana (or that of any drug) at a later date.

But opponents argued that passage of the measure could encourage marijuana use, and do little to help the city's jobless situation.

"I submit that manu fo those people can't get a job because of the ocnidtion that they have gotten into already," said member Jerry A. Moore Jr. (R-at large), a Baptist minister and an opponent of the bill.

"You go up to 14th and T Street (NW) and daily you see them standing there on the corner in such large numbers that it looks like some kind of convention, waiting for the man to come, young black men and women who have been very little use to our society except as welfare cases and medical records.

"I've been to 14th and T and seen some other things, Reverend Moore," cuntered Clarke, who is chief sponsor of the bill, "I've seen some win. I've seen winos. I've seen some stores and dealers without stores.

"And yet when the liquor dealers come here, as they have a right to come here." Clarke said, "I haven't heard the intensity of objection . . . with respect to the subject."

"We talk about the people at 13th and 14th and 15th and U Streets," said member Arrington Dixon (D-four), a backer of the bill. "There are many organizations that operated in those communities for years that sucked, sucked on those poor people that are affected by the present law. And we're going to perpetuate that.

"Those folks that are there are 14th and T have been there ever since I've been in this community and the institutions were around during that period, yet I haven't seen any remedies brought forth."

Voting in favor of the measure in addition to Clarke and Dixon were John A. Wilson (D-two), Polly Shackleton (D-three), Wilhelmina J. Rolark (D-eight), Marion Barry (D-at large) and Hilda Mason (Statehood-at large). Joining Jerry Moore in opposition were Douglas E. Moore (D-at large), William R. Spaulding (D-five), Nadine P. Winter (D-six), Willie J. Hardy (D-seven) and Council Chairman Sterling Tucker (D).