A North Carolina tobacco farmer sentenced last week to six years in prison for driving his tractor onto the Mall and threatening to detonate explosives had his sentence cut abruptly yesterday in one of the first cases to get a review after a Supreme Court ruling.

"Hallelujah!" Dwight W. Watson exclaimed in court yesterday morning after a federal judge resentenced him to 16 months. Watson has been locked up for 15 months and 11 days since his arrest in March 2003. With credit for good behavior, the man who waged a 47-hour standoff with police could be eligible for release this week.

A Supreme Court decision last Thursday raises questions about the legality of tens of thousands of other criminal sentences handed down across the nation. Watson was sentenced just one day before the high court's ruling, which said that juries -- not judges -- must decide on facts that are used to increase sentences beyond the maximum ranges called for in sentencing guidelines.

U.S. District Judge Thomas Penfield Jackson went beyond the 16-month term set by the guidelines in Watson's case, saying that his menacing conduct and the havoc he caused called for a much longer sentence. Yesterday, acting on a motion filed by the defense, Jackson said he had no choice but to reduce Watson's punishment.

Watson is the third criminal in the country, after cases this week in Utah and Arizona, to receive a reduced sentence since the high court ruling, according to prosecutors.

"The Supreme Court has told me that what I did a week ago was plainly illegal," Jackson told Watson. "By my count, Mr. Watson, you're a free man in a few hours."

But Watson remained at the D.C. jail as the court's probation department and the Federal Bureau of Prisons did their own reviews before setting a release date. And early last night, after all-day meetings and consultations with top officials at the Justice Department, federal prosecutors filed papers with an appellate court to prevent Watson's release and to argue for the longer sentence. They argued that Watson remains a danger to the public.

After making the trip to Washington from his Whitakers, N.C., farm, Watson, 51, set the city on edge March 17, 2003, when he drove his John Deere tractor into a Constitution Gardens pond. He complained through a bullhorn about bankrupt tobacco farmers and warned authorities that he had "organophosphate bombs." His actions led officials to close several nearby government offices, and road closings disrupted four consecutive rush hours. Police officers, including SWAT teams, surrounded the area until Watson gave up.

During his trial in September, Watson testified that he meant no harm and that he only wanted to highlight the plight of tobacco farmers and the dangers of pesticides. He had no bombs, only a couple of cans of bug spray.

The jury convicted him of making threats and destroying government property. Prosecutor Jay Bratt argued at sentencing that Watson's crimes called for a significant prison term, noting that the standoff occurred at a time when Washington was under an elevated terror alert and on the eve of the war with Iraq.

Jackson agreed that Watson deserved a stiffer penalty because he engaged in conduct that showed intent to harm, caused substantial disruption to the public and government, made more than two threats and gave false statements at his trial.

The judge told Watson last week that his protest terrified people and that the city regarded him as "a one-man weapon of mass destruction."

But less than 24 hours after those stern words, the Supreme Court issued a 5 to 4 decision in an unrelated case with parallels to Watson's. The high court said a trial judge in Washington state violated the Constitution by sentencing a convicted kidnapper, Ralph H. Blakely Jr., to 90 months in prison rather than the 53-month maximum set by state law. The high court said the judge had increased the sentence based on facts that were not ruled upon by a jury.

Jackson hastily scheduled yesterday's hearing, during which he said he felt that he had acted in the same manner as the Washington state judge. His ruling stunned the small audience of prosecutors and probation officials and gratified Watson's public defenders, A.J. Kramer and Erica J. Hashimoto.

Watson's brother, George Watson, said in an interview that he thought someone was playing a trick on him when he received the news about the new sentence -- until he confirmed it with the lawyers.

"I've been walking on a cloud ever since," he said. "There's not a person down here who doesn't think he has already more than paid for what he did."

George Watson said that he salutes Jackson for reversing himself and that it was "disappointing" that prosecutors want to fight his brother's release.

"The only thing I can come up with is they hate to lose," he said.

The Supreme Court decision, in Blakely v. Washington, has been a bombshell for federal prosecutors across the country, and the Justice Department is working quickly to draft a national strategy for handling a potential cascade of defense motions to set aside or reduce sentences.

Legal specialists said the ruling seems destined to change the structure of plea bargains and trials in federal courts, to affect sentencing guidelines in about a dozen states and to have at least some effect on any trial in which a judge is allowed to add time to a sentence based on evidence not presented to the jury. That has been a key part of federal sentencing and, to varying degrees, of sentencing in state courts across the country.

The defendants who stand to benefit are those who have not been tried or who are within the time limit for appeals, according to the specialists.

Justice Antonin Scalia wrote for the majority that exceptional circumstances must be presented and proved to a jury or the sentence is unconstitutional.

In a dissent, Justice Sandra Day O'Connor warned that the practical consequences of the decision "may be disastrous" and that the majority was ignoring "the havoc it is about to wreak on trial courts across the country. . . . Over 20 years of sentencing reform are all but lost, and tens of thousands of criminal judgments are in jeopardy."

That process began immediately, starting with a convicted child pornographer in Utah.

Brent Coxford pleaded guilty to taking pornographic pictures of his 9-year-old daughter, a crime that would have required him to serve 121 to 151 months in prison. But a pre-sentence report after his guilty plea indicated that he also had taken pornographic pictures of another child, an offense that would have required the judge, under federal guidelines, to impose a longer sentence, of 151 to 188 months.

On Tuesday, U.S. District Judge Paul G. Cassell wrote that he "reluctantly agreed" that such added prison time was now unconstitutional. He sentenced Coxford to 148 months and warned that the impact of the Supreme Court decision was "potentially cataclysmic."

Harvard Law School professor William J. Stuntz said the Supreme Court ruling "invalidates the large majority of federal sentences," but he said the final outcome was not yet clear.

"The court has said the current set of rules is not good, but it didn't specify what the new rules should be," Stuntz said. "For a while, there's going to be a tremendous amount of legal uncertainty, and that's a very bad thing in a criminal justice system."

According to the National Center for State Courts, the states most likely to be affected by the ruling are Alaska, Arkansas, Delaware, Florida, Michigan, Minnesota, North Carolina, Oregon and Washington. Other states, including Virginia, have voluntary guideline systems that also may have affected sentences. D.C. Superior Court might be affected as well.

Staff writer David Nakamura contributed to this report.

A federal jury convicted Dwight W. Watson of making threats and destroying government property.