D.C. Mayor Marion Barry said yesterday that he plans to ask the City Council today to reduce the maximum penalty for possession of marijuana from one year in jail to 90 days for persons without a prior narcotics record.

But prosecutors say that what at first glance appears to be a relaxation of the city's marijuana law may in fact mean tougher prosecution for offenders caught with small quantities of the drug, since the change would eliminate defendants' rights to a jury trial if charged with the misdemeanor offense.

The new legislation is one of several bills city officials have proposed as a means of eliminating delays in D.C. Superior Court and relieving overcrowding in the city's prison facilities.

The bill would also reduce from one year to 90 days the penalty for first convictions of numerous other crimes, such as destruction of property worth less than $200, writing bad checks less than $100, maintaining a gambling premises, simple assault and credit card fraud of less than $250.

Prosecutors have been pushing for the reduced sentences for years, arguing that judges rarely give out jail terms to first offenders convicted of misdemeanor crimes but that the threat of jail time causes many defendants to resist plea bargaining and take their cases to juries, using up valuable prosecutorial resources. Many judges also favor the reductions, saying they frequently try cases they feel are insignificant.

The mayor's proposed reduction in the penalty for marijuana possession would not affect the penalty for possession of other drugs, such as heroin, which would remain punishable by up to a year in prison for first offenders and up to two years in jail and fines of $2,000 for repeat offenders.

Prosecutors said the reduced penalties will mean tougher prosecution. Currently, many first offenders charged with marijuana possession are not prosecuted because juries are reluctant to bring convictions on the charge, authorities said. Under the new bill, defendants will not be permitted to ask for a jury trial and, unless they plead guilty first, will go to trial before a judge, a process referred to by many court officials as "a slow guilty plea" because of the traditionally high rate of conviction.

Trials before judges require fewer preliminary court hearings, which saves prosecutors' time. The new sentencing bill was part of a package of proposed legislation sent in July to a federal judge as a sign that city officials are trying to reduce overcrowding at D.C. Jail and Lorton Reformatory.