Sen. Arlen Specter (R-Pa.), chairman of the Senate Subcommittee on the District of Columbia, said yesterday that he believes that some potentially violent criminals are not serving prison sentences in the District because there is no room for them in the city's severely overcrowded prison system.

Specter's comments came after prison specialists, law enforcement officials and victims testified before the subcommittee on the subject of criminals who commit additional crimes while free on probation.

Joan Petersilia, a criminologist with the Rand Corp., a nonprofit research organization, testified that a study it conducted showed that "routine probation is not a good bet for most convicted felons."

Petersilia said the study tracked 1,672 persons convicted of felonies in California in 1980 who were released on probation instead of serving prison terms. She said that 65 percent were rearrested, 51 percent were convicted and 34 percent were sentenced to jail or prison for new crimes.

She said that of those convicted again, "nearly 20 percent were found guilty of serious, violent crimes -- homicide, rape, assault and armed robbery."

About 25 percent of the 1,672 persons on probation "looked, in a statistical sense, identical to the people who went to prison," but were not sentenced because of a lack of space, she said.

"My sense is that people nationwide are being put on probation because there's no room in the jails," Specter said, "and that situation exists in the District of Columbia."

However, Judge Fred B. Ugast, chief of the criminal division of D.C. Superior Court, said that he did not believe overcrowding at the D.C. Jail and Lorton Reformatory discouraged judges from sentencing criminals to these institutions. In his own case, Ugast said, he just "cringed" and sentenced them.

Ugast said that in the last three weeks, the court had implemented new procedures that combine a probation revocation hearing and a hearing on probable cause on any new charge before the same judicial officer. Meanwhile, defendants are held for up to five days without bond.

Ugast said that 44 persons had appeared under the new hearing procedures since they went into effect Jan. 28. Of this number, he said, 36 were later held without bond because they were deemed likely to flee or posed a threat to themselves or the community.

Officials of the Georgia Department of Offender Rehablilitation testified that stringent probation surveillance accounted for that state's low rate of repeat offenders. They said that only 9.6 percent had probation revoked for committing new crimes.

U.S. Attorney Joseph E. diGenova, who testified in support of stricter probation surveillance in the District, said there is a "core of recidivists . . . who don't give a whit about public safety" who are causing their victims to live in fear.

Citizens and law enforcement officials have "bent over backward to be indulgent with criminals," diGenova said after the hearing. "Now we're starting to think about the victims and the repeat offenders."