Mayor Marion Barry's criminal justice task force has recommended giving the mayor the authority to commute prison sentences, primarily as a tool for reducing what the report called "dangerously overcrowded" prison conditions.

In a preliminary report scheduled to be delivered to the mayor today, the 35-member task force, part of Barry's transition team, said the D.C. government should have a contingency plan to reduce overcrowding in the D.C. Jail and Lorton by "judiciously applying executive authority to grant clemency or pardons."

"Such authorization has the advantage of reducing the incarcerated population while maintaining the deterrent effectiveness of institutionalization . . . ," the task force said in a portion of a draft report obtained by The Washington Post.

The task force said that arrests for serious crimes increased by 34 percent between 1978 and 1981, while the number of persons sent to penal institutions increased by 52 percent in those years.

Moreover, the report said, the parole board recently instituted new guidelines that are expected to make it tougher for prisoners to get out on parole, and city judges are handing out stiffer sentences for most serious crimes.

City Council member and Chairman-elect David A. Clarke, who now heads the judiciary committee, said he might support the proposal on the grounds that governors in practically every state have the power to commute sentences--but not as a tool for alleviating overcrowded conditions.

"It's a standard power of chief executives in every state and it ought to exist in the mayor," Clarke said. He added that Congress would have to approve legislation granting the mayor that power.

Lawrence Doss, a partner in the accounting firm of Coopers & Lybrand and chairman of the task force, declined yesterday to confirm or discuss the proposal.

"I'm not at liberty to discuss proposals until we share them with the mayor," Doss said. "Our basic mission is to come up with recommendations to increase the efficiency of the criminal justice system."

Doss said the task force and its subcommittees have held 15 to 20 sessions during the past month to prepare the report. But another member of the task force, who asked not to be identified, said members didn't have enough time to produce fully developed proposals.

"It was more like a seminar where people expressed their ideas, but there was no in-depth consideration of data by the task force itself," the member said.

The draft report also includes proposals to: increase the number of prisoners who could be freed on work-release or furlough programs; realign present police disrict boundaries; defer purchase of new police cars and extend the life of current ones by spending more for maintenance; and set up a centralized criminal justice planning unit.

Annette Samuels, Barry's press secretary, said yesterday the mayor will spend part of the weekend reviewing reports from Doss' group and seven other transition task forces that were appointed to recommend new policies for Barry's next four years in office.