The Justice Department is pressing Congress to act immediately to prevent thousands of crack cocaine offenders from being released from prison early in response to a federal commission’s decision making retroactive sentence reductions possible for such criminals.

Some Justice Department officials and senators are warning that releasing the offenders would cause a surge in violent crime.

“March 3rd the retroactivity goes into effect. We’re on a very short time window right now,” the United States attorney from Charlotte, N.C., Gretchen Shappert, told a Senate subcommittee.

Ms. Shappert, the official Justice Department representative at the hearing, said up to 20,000 prisoners, or 10% of the federal prison population, could ask judges to shorten their sentences under the change adopted in December by the Sentencing Commission. The prosecutor said a flood of applications could be a time-consuming distraction for prosecutors struggling to keep up with their current workload of trials and investigations.

“Files have been archived. Witnesses are gone. Agents have retired,” Ms. Shappert said.

A Florida defense lawyer who also testified yesterday, James Felman, questioned Attorney General Mukasey’s warning last month that the sentence reductions would hit inner-city communities with “a sudden influx” of ex-offenders likely to commit new crimes. The defense attorney, who was co-chairman of an American Bar Association panel on the issue, noted that some 650,000 people get out of prison each year.

“For the attorney general of this nation to put our people in fear over the release of 1,600 people, knowing that otherwise 650,000 people are going to be released, is truly disappointing,” Mr. Felman said. “We’re not talking about releasing 20,000 of them now. We’re talking about releasing 2,000 or less now. So we’re actually talking about 1% or less of the prison population that would be released at any given time.”

Senator Sessions of Alabama, a Republican, did not express opposition to retroactive sentence reductions, but he warned that the consequences could be devastating for certain neighborhoods and for individuals.

“I don’t know how many people will die as the result of a mass release of 25% of the federal penitentiary but some will, because a lot of these people will go back to this and get involved in violence and kill somebody,” Mr. Sessions said. “I think we need to be more realistic here.”

Civil rights groups, defense lawyers and others have complained for years about the racial impact of federal laws which treat possession or distribution of crack or “rock” cocaine 100 times more seriously than an equivalent weight of powdered cocaine. The vast majority of crack offenders are African American, while those arrested for crimes involving powdered cocaine tend to be white or Hispanic.

In May, the Sentencing Commission proposed a reduction in the recommended sentences for crack offenders. The change went into effect in November when Congress took no action to block it.

Ms. Shappert insisted yesterday that crack crimes deserve to be treated more seriously. “Whereas cocaine powder destroys an individual, crack cocaine destroys a community,” she said. “A variety of factors fully justify higher penalties for crack offenders.”

A federal judge, Reggie Walton, expressed doubt that crack was an unusual scourge. “There are a lot of drugs that have an impact on communities,” he said.

Judge Walton, who oversaw the trial of Vice President Cheney’s aide I. Lewis “Scooter” Libby, noted that when sentences were reduced for LSD and marijuana, drugs mainly used by whites, the changes were made retroactive. He said African Americans would be rightfully distressed if crack were treated differently. “I don’t think anybody you’d talk to would tell you that I’m lenient when it comes to crime, but I believe in fundamental fairness,” he said.

Mr. Mukasey has suggested that Congress pass a bill that would require violent convicts to serve their full sentences, but allow non-violent offenders out early. However, Judge Walton said judges needed to make those decisions. “That’s very difficult to effectuate through legislation,” he said.

Most senators have not taken clear positions on the retroactivity issue, so it is unclear whether legislation on that will advance before March 3rd. Among Democratic presidential candidates, Senator Clinton has said she has “problems” with retroactivity, while Senator Obama of Illinois said he favors it. The Republican frontrunner, Senator McCain of Arizona, did not respond to an inquiry on the issue yesterday.

The chairman of the subcommittee, Senator Biden of Delaware, a Democrat, said he favors eliminating the 100-to-1 disparity altogether, but would agree to simply reduce it if colleagues preferred that course. “I would rather get something good done than nothing done at all,” he said.

Mr. Sessions, a former prosecutor, said a minimum mandatory sentence of five years for possession of five grams of crack was glaringly unfair. “I think that’s an excessive sentence,” he said.