The Prince George's County attorney has proposed a plan to ease overcrowding of the county jail by releasing early on a regular basis some inmates charged with lesser offenses.

But the county police and the sheriff's office have expressed opposition to the plan, saying they believe the new policy will result in part in inmates not showing up for trial.

The preliminary plan to keep the jail population at 425 or less was outlined by County Attorney Thomas P. Smith in a letter delivered this week to U.S. District Court Judge Frank Kaufman. Last year, Kaufman issued a consent decree ordering the county to relieve security and overcrowding problems at the detention center in Upper Marlboro, which often has held 50 to 100 more prisoners than capacity.

The new plan will formalize releases similar to those made last month when District Court Judge Graydon S. McKee III freed 27 inmates held on misdemeanor charges.

The program will require cooperation from the state's attorney's office and county judges. In addition to lowered bond for some offenders, it includes a pre-trial release program, speedy sentencing following trial and prompt transferral of state prisoners to state facilities.

Inmates charged with crimes such as shoplifting or disorderly conduct would be eligible for early release. Col. Ernest J. Zaccanelli, chief assistant county sheriff, said yesterday he believes that many of these people will not return on their appointed court date. Therefore, Zaccanelli said, his office will have to issue more bench warrants and find the no-shows.

County police chief John E. McHale Jr. agreed. "It's a Catch-22--the public wants us to do a job--we do the job and make arrests, then the federal judge tells us to turn them loose . . . It's frustrating," he said. McHale said he believes that the new jail, scheduled to be ready in 1985, is the only answer to the overcrowding problem.

Catherine Bouchard, a Circuit Court law clerk, has been named coordinator of the proposed pretrial release program. An example, she said, of someone she would recommend for early release or disposition is a District man who has been in the jail for several weeks, charged with not paying a $4 cab fare, and who cannot pay his $200 bond.