A sweeping new method of determining sentences for persons convicted of federal crimes, which takes effect today, could radically alter the everyday workings of the federal judiciary and crowd court calendars with legal challenges for years, some criminal experts say.

Meanwhile, no one in the criminal justice system appears prepared for the change in procedures, which are designed to eliminate the current wide disparities in punishment imposed for similar offenses.

Although the U.S. Sentencing Commission has been working for three years on the so-called "sentencing guidelines" and the concept has been under consideration for a decade, judges received the final guidelines only early last week. Prosecutors here got them Friday and they still are not available to the public, including defense attorneys.

Because of the expected impact of the guidelines -- particularly in the areas of plea agreements, sentence computations and appeals of sentences -- judges, prosecutors, defense attorneys and even the commission that drafted the new guidelines tried to get Congress to delay their effective date, but without success.

Under the old procedures, sentencing was left up to the judge in the case. Prosecutors could recommend sentences to the judges, and the U.S. Probation Service made a recommendation based on its own investigation of the crime and the defendant.

But the judge had complete discretion to impose any legal sentence, ranging from no prison time or probation to the maximum amount of incarceration.

The judge also set the amount of fines.

The new guidelines permit judges little latitude in the length of sentences they may impose, and place similar constraints on the fines they may levy.

Under the new system, the charges ultimately determine the person's sentence, so prosecutors will have to consider sentencing from the very beginning of the charging process.

And because prosecutors will be considering sentencing that early, defense attorneys will have to be involved in the process then, too.

"You have a much more important role than the defense lawyer has ever had before in being a very strong advocate," said Laurie Robinson of the American Bar Associaton's Criminal Justice Section.

"The system is not prepared for it," Robinson said.

"It's going to be a disaster," a Justice Department official predicted last week.

"Implementation is going to be very messy, at least for a year," another official said.

"Total chaos may be a little too strong," said Paul Martin, spokesman for the sentencing commission. "It's, of course, going to be a big change in the system, but it's going to be a somewhat gradual change rather than an overnight, abrupt, day-and-night change." Martin added that the guidelines apply only to crimes committed today or afterward.

But prosecutors and defense attorneys see it a bit differently. "We can arrest somebody on drug charges Sunday, and be offering them a plea agreement by Tuesday," said Assistant U.S. Attorney Thomas Zeno, who is in charge of training prosecutors here.

"Sentencing will no longer be a consideration only at the end of the criminal process," said Assistant U.S. Attorney Mark Biros, who teaches criminal procedure at Georgetown University Law School. "Sentencing will be an important consideration at every stage of the process."

Under the new guidelines, which are mandatory, there is no parole and the amount of "good behavior" time used routinely to reduce sentences has been sharply limited. Probation is allowed in few circumstances. As a result of the new guidelines, which employ a mathematical formula to calculate appropriate sentences, some experts estimate the federal prison population will increase 50 percent or more in the next five years.

Despite the dire predictions from prosecutors and others, U.S. Appeals Judge Stephen G. Breyer, a member of the sentencing commission, says he believes new guidelines fulfill the mandate to bring rationality and uniformity to the "absolutely dark, mysterious process of sentencing."

"In the future, the sentence the judge imposes is basically the sentence the offender will serve," Breyer said. "It's honesty in sentencing."

Still, Breyer believes the prison population will increase "only slightly," perhaps about 6 percent.

Other legal experts believe there are constitutional problems with the new sentencing procedure and expect a legal challenge to the whole process will be filed soon. They argue that under the new guidelines judges have acted as legislators, breaching the separation of powers mandated in the Constitution.

But, for now, the criminal justice system must grapple with the implemention of the guidelines.

Zeno and others believe there will be three problem areas in putting the guidelines into effect: plea agreements, sentence computations and appeals of sentences. All are expected to be major.

More than three-quarters of all federal criminal defendants enter pleas rather than go to trial, usually because they believe it will result in a much-reduced prison sentence.

Under the new guidelines, persons charged with similar offenses will face roughly the same sentence whether they plead guilty or are convicted as a result of a trial. The maximum reduction allowed for those who plead guilty is about 25 percent, and even that is not guaranteed.

"I think it's going to be a lot harder to persuade defendants it's in their interest to plead guilty," said a defense attorney, who asked not to be identified.

"If they are facing roughly the same prison term, it might be worth it to go to trial," he said, noting that a great increase in the number of trials probably would prompt prosecutors to dismiss some cases.

"In terms of pleas, nobody knows what the consequences are going to be," said a local prosecutor, who said that if more defendants choose to go to trial it will strain the criminal justice system.

"No one is looking forward to this," he said. "It will make plea negotiations much more complicated."

In Minnesota, which imposed a similar sentencing method a decade ago, the percentage of guilty pleas has remained about the same. Officials think the same may happen in the District eventually, but they say they do not know what the short-term effect here will be.

The second logjam is expected to center on the actual sentencing hearings. Under the new guidelines, the length of imprisonment is determined by the "offense level" assigned to each crime. But the mathematical formula provides for various adjustments, up and down, according to other factors, such as whether the defendant played a "minimal" role in the crime and whether the person cooperated with the investigation or obstructed it.

If there is a dispute over such factors, the sentencing judge will have to decide which are applicable. That may require full-scale hearings on those matters, even in cases in which the defendants pleaded guilty.

"The guidelines have opened up a Pandora's box of factual and legal issues," another prosecutor said.

The third major impact of the guidelines is expected to be on the appeals courts, rather than the district courts where sentences are imposed.

Under current procedures, people who plead guilty generally cannot appeal their sentences. But the sentencing guidelines permit those who believe their sentences do not conform to the guidelines to appeal.

At least in the beginning, Zeno thinks, many defendants will challenge their sentences. With the federal court in Washington averaging more than 400 criminal cases a year -- and twice that many separate defendants -- sentence appeals could clog the already overloaded U.S. Court of Appeals.

"We will have to come up with some way to expedite appeals," U.S. District Judge Aubrey E. Robinson Jr., chief judge in the District, said last week.

Robinson plans a training program for judges in the District and similar sessions for defense attorneys.

The U.S. attorney's office in the District began training sessions for its prosecutors last week, as did the local probation office. And the American Bar Association is setting up a massive training program by satellite to reach lawyers across the country. The program starts in January.

"We're hoping for the best," said U.S. Attorney Joseph E. diGenova. "Congress has spoken."Staff writer Ruth Marcus contributed to this report.