A mandatory sentencing measure passed by District of Columbia voters last year goes into effect Tuesday, requiring for the first time that people found guilty here of certain drug offenses or of committing violent crimes with guns receive mandatory minimum prison terms with no chance of parole.

City officials say they are unable to predict exactly what will happen in the city's already-overloaded court and prison systems when the tough new law begins to work.

Law enforcement officials say they believe the statute, if enforced fully, could swamp the court with additional trials by eliminating incentives for defendants to plead guilty. Also, they say, mandatory sentences could result in a dramatic upsurge in the number of people behind bars at the city's overcrowded corrections facilities.

City Council member John Ray (D-At Large), who led supporters of the measure, disputed both points. "I think initially it will mean an increase," Ray said in a recent interview. But increases in trials and in the number of inmates will eventually diminish, he predicted, because the law will have a deterrent effect on crime and reduce the number of criminal cases overall.

Thirty-seven states have passed some form of mandatory sentencing.

In Philadelphia, the number of jury trials has more than doubled since last June when a new Pennsylvania law established a five-year mandatory minimum sentence for crimes committed with firearms and for repeat violent offenders.

Philadelphia District Attorney Edward G. Rendell, a strong supporter of mandatory sentencing, estimated that in his city the law eliminated 95 percent of plea bargains between prosecutors and defendants in cases where it applied. The state, he said, was required to transfer four additional judges to city courts to handle the increased number of trials.

Rendell said that offenders who previously were released on probation now go to prison, and he expects a massive increase in the prison population.

Rendell noted that unlike the District, Pennsylvania passed a $112 million bond issue for construction of 2,800 additional prison cells at the same time the mandatory sentencing law was passed.

District officials are under a federal court order to place a limit on the number of inmates at Lorton Reformatory. The inmate population there has already reached maximum level and officials are still seeking a solution to overcrowding.

"Without the bond issue," Rendell said, "it would have been ludicrous" to pass the mandatory sentencing legislation in Pennsylvania. "Otherwise our bad guys would go in the front and other bad guys would go out the back."

Corrections officials here estimated that the law will add 250 to 300 inmates to the prison at a cost of more than $15,000 each. Ray said those estimates are too high.

"Corrections doesn't have the foggiest idea," he said.

Law enforcement officials said judges and prosecutors may seek ways of avoiding the new measure, such as bringing or reaching plea agreements when possible on charges not covered by the legislation.

U.S. Attorney Stanley S. Harris, along with local judges, most City Council members and civil liberties groups, opposed the mandatory sentencing statute. He unsuccessfully sought changes in it that would allow greater flexibility for judges and prosecutors. Last week, after his last effort to win amendments failed, he said: "I intend to enforce the law."

D.C. Superior Court Chief Judge H. Carl Moultrie I said he believed the new law is likely to result in increased jury trials. "Do you believe that . . . our prosecutor will get anybody to plead to an armed robbery when there is a must five years in jail?" he asked.

District voters passed the mandatory sentencing measure in September 73 percent to 27 percent after efforts to push it through the City Council failed.

In March the council agreed to delay implementation of the law for 90 days, after law enforcement officials voiced concern that technical aspects within the legislation needed to be ironed out before the statute could be applied in court