WASHINGTON - I. Lewis Libby Jr., once one of the most powerful men in government as Vice President Dick Cheney's chief of staff, was sentenced today to two and a half years in prison for lying to a grand jury and F.B.I. agents who were investigating the unmasking of a C.I.A. operative, Valerie Wilson, during a fierce debate over the war in Iraq.

Federal Judge Reggie B. Walton also fined Mr. Libby $250,000 after declaring that there had been "overwhelming evidence" of Mr. Libby's guilt on the four counts - one each of obstruction of justice and giving false statements, and two of perjury. He was convicted on March 6.

Judge Walton did not set a date for Mr. Libby to report to prison. The judge said at first that he saw no reason for the defendant to remain free pending appeal, but he later agreed to accept briefs on that issue and rule later.

Before he was sentenced, Mr. Libby stood before the judge and expressed thanks for what he described as unfailing courtesy from the people who work in the federal courthouse. Then, in a soft, calm voice, he said, "It is, respectfully, my hope that the court will consider, along with the jury verdict, my whole life. Thank you, your honor."

The judge said he appreciated the fact that Mr. Libby had worked tirelessly for his government and country, at considerable financial sacrifice. But "we expect and demand a lot" from people in government, the judge said.

Sadly, he said, Mr. Libby "took a course that seems to be contrary to everything he's done in his life."

There is no provision for parole in the federal prison system. If Mr. Libby's conviction and sentence are sustained on appeal, he will have to serve nearly all the 30 months imposed by Judge Walton, though he could win some reduction for good behavior.

Mr. Libby and his lawyers left the courthouse through a pack of journalists and a few hecklers, who shouted, "Go to jail right now! Take Cheney with you!" and other imprecations.

There has been widespread speculation that President Bush might pardon Mr. Libby, but the White House said today that the president would take no action while appeals are pending. Mr. Bush himself is in Europe; a White House spokeswoman, Dana Perino, said Mr. Bush felt "terrible" about the ordeal of Mr. Libby and his family.

The sentencing followed a long argument that covered highly technical issues that govern federal sentencing guidelines, and then delved into whether and how a person's government service should matter when he stands before the bar of justice.

Mr. Libby's chief lawyer, Theodore V. Wells Jr., asked Judge Walton to place his client on probation, or at least place him under home confinement or in a halfway house, rather than send him to prison.

A person's status and wealth should not matter in sentencing, Mr. Wells said, but "the good works and the good deeds a person has done" surely do.

The lawyer alluded to the dozens of letters, some from high-ranking public officials, written on Mr. Libby's behalf and attesting to his tireless and selfless service.

Yes, Mr. Wells conceded implicitly, Scooter Libby, as he has been known since childhood, came from a privileged background. But there are privileged people "who don't do squat for their country, who don't help others," Mr. Wells said, in contrast to the "exceptional public service" of his client.

Mr. Wells said Mr. Libby had already been punished, through "public humiliation," hate mail, the virtually certain loss of his license to practice law and the desire of some people to make him "the poster child" for all that has gone wrong with the Iraq war. Mr. Libby's family has also suffered, the lawyer said.

"He has fallen from public grace," Mr. Wells said, his voice dropping to a hush. "It is a tragic fall, a tragic fall."

But the prosecutor, Patrick J. Fitzgerald, argued that the defendant had shown "absolutely no contrition," and that the court needed to send a message that lying in a criminal investigation can never be excused.

"Truth matters," Mr. Fitzgerald said. "The whole system depends on that."

Mr. Libby's conduct was inexcusable, the prosecutor said, because as a lawyer he is "educated and experienced in these matters."

Nor was the defendant's lying a spur-of-the-moment decision, Mr. Fitzgerald went on. Rather, he said, Mr. Libby lied repeatedly over a period of weeks, rejecting numerous opportunities to change his account and avoid charges.

As for Mr. Libby's long record of public service, Mr. Fitzgerald said, "We cannot create a special category of people" who are treated leniently just because they are in government.

Mr. Fitzgerald argued that it does not really matter than no one has been prosecuted for actually leaking the C.I.A. affiliation of Ms. Wilson, also known by her maiden name, Valerie Plame. The defendant knowingly impeded the inquiry, Mr. Fitzgerald said, and made it difficult for investigators to determine "who's telling the truth in this house of mirrors he created."

The episode that brought down Mr. Libby began four summers ago, when the columnist Robert D. Novak disclosed that Ms. Wilson - whose husband, the former diplomat Joseph D. Wilson IV, publicly expressed doubts about the Bush's administration's rationale for going to war in Iraq - worked for the C.I.A.

Mr. Fitzgerald was named a special counsel to look into who had disclosed Ms. Wilson's name to Mr. Novak, which, depending on the circumstances, may have been a crime. No one was ever prosecuted for that offense, and Mr. Libby was ultimately undone by his statements to grand jurors and F.B.I. agents.

Essentially, Mr. Libby claimed that he learned of Ms. Wilson's identity from reporters. But Mr. Fitzgerald subpoenaed several reporters to rebut the defendant's claim. Rather, the reporters said, it was Mr. Libby who told them, not the other way around.

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