Retired Supreme Court Justice Thurgood Marshall, a relentless voice for minorities whose six-decade legal career was emblematic of the civil rights revolution, died yesterday of heart failure.

He was 84 years old and had been retired since June 1991. Marshall had been in failing health in recent months. He died at the National Naval Medical Center in Bethesda, where he had been since Thursday. He had planned to administer the oath of office to Vice President Gore last Wednesday, but could not because of his condition.

Marshall, who was born in Baltimore the son of an elementary school teacher and yacht-club steward, went on to become one of the most important figures in civil rights history, first as a lawyer for the National Association for the Advancement of Colored People (NAACP) and then as the first black Supreme Court justice. He was known for both his sense of humor and his impatience over the ongoing struggle of blacks in America.

"He was somebody who had absolutely no sense of his own importance," said Louis Michael Seidman, a former Marshall clerk who is now a Georgetown University constitutional law professor. "He held an unusual combination of reverence for the American justice system and a realization that his people were excluded."

In 1967, President Lyndon B. Johnson appointed Marshall to the court. During his 24-year tenure, he was the only black justice. He was replaced by Clarence Thomas, also a black man, but one who adopted a judicial approach that is the opposite of Marshall's liberalism.

Marshall's record on the court was consistent: Always the defender of individual rights, he sided with minorities and the underprivileged; he favored affirmative action and supported abortion rights; and he always opposed the death penalty.

But he was not the liberal leader that retired Justice William J. Brennan Jr. once was. He did not strive for consensus, and as a result was the author of few significant majority opinions.

In a statement, President Clinton said Marshall was "a giant in the quest for human rights and equal opportunity in the whole history of our country."

Chief Justice William H. Rehnquist said Marshall will be remembered as much for his work before coming to the court as afterward, for "his untiring leadership in the legal battle to outlaw racial discrimination."

Before Marshall joined the court, he had distinguished himself as the country's first black solicitor general, serving in that post from 1965 to 1967 and taking a lead in promoting the Johnson administration's civil and constitutional rights agenda.

Marshall came to national prominence as the chief lawyer for the NAACP Legal Defense and Educational Fund, when he argued a series of 1954 school desegregation cases known collectively as Brown v. Board of Education. The Supreme Court ruled in those cases that segregation in public schools was unconstitutional.

As a lawyer, Marshall also took the lead in litigation that ended white-only primary elections and explicit racial discrimination in housing contracts.

His greatest cause was defendants' rights, and when he left the court two years ago, he was the last of the justices to oppose the death penalty.

People close to him said frustration with the court's conservative turn in recent years prompted his retirement.

But at a news conference at the time, Marshall blasted suggestions that his retirement stemmed from anger about the future of the conservative-dominated court.

"What's wrong with me?" Marshall said impatiently. "I'm old. I'm getting old and coming apart."

Such was the style of a man who could be eloquent or, when he wanted, slip into slang and black dialect. When he was asked what he was going to do in retirement, he said, "Sit on my rear end."

He was 6-foot-2, a physically imposing man who always appeared to be coming out of his black robes, and had a distinctive gravelly voice. He said he wanted to be remembered this way: "That he did what he could with what he had."

Marshall's roots were unlike those of any other justice before him.

He was born July 2, 1908. The great-grandson of a slave brought to America from Africa's Congo region, Marshall was named after a paternal grandfather, who had chosen the name "Thorough Good" for himself when enlisting in the Union army during the Civil War. Marshall later changed it to Thurgood.

His mother was an elementary school teacher and his father a steward at an all-white yacht club on the Chesapeake Bay.

Marshall attended Douglas High School in Baltimore, working as a delivery boy for a women's store after school.

He later confessed to having been a bit of a cutup in high school and college. He recalled that in high school he often was punished by being sent to the basement and forced to memorize "one paragraph of the Constitution for every infraction. . . . In two years, I knew the whole thing by heart," he said.

Marshall attended the all-black Lincoln University in Pennsylvania, earning money for tuition by waiting tables.

He obtained his law degree from Howard University in 1933, graduating first in his class.

Marshall attributed his interest in law to "arguing with my dad. We'd argue about everything." He also credited his father with instilling in him a fighting spirit. "Son," he once recalled his father saying, "if anyone ever calls you a nigger, you not only got my permission to fight him, you got my orders to fight him."

Marshall remembered carrying out those orders one time when, as a delivery boy, he accidentally brushed against a woman on a Baltimore trolley car because he couldn't see over a stack of hat boxes he was carrying. A white man called him "nigger" and Marshall took him on.

Marshall began practicing law in Baltimore after graduating from Howard. One of his first civil rights cases was a successful effort to gain admission for a young black man to the University of Maryland Law School.

Three years later, he was hired as an assistant to the national counsel for the NAACP and two years later became chief counsel.

In late 1939, he created the NAACP Legal Defense and Educational Fund, and as its head from 1940 to 1961 he worked within the legal system to improve minority rights.

Traveling around the country, he won dozens of civil rights victories. He recalled in recent years how he was often run out of town by whites who despised his work for black liberation.

Marshall won all but three of the 32 cases he argued before the Supreme Court, including the 1954 Brown ruling. That landmark decision ended "separate but equal" school systems. He achieved Brown through a series of court cases over several years, methodically dismantling the foundations of segregation.

He also was at the lead in the integration of the Little Rock, Ark., Central High School in 1957, as well as crafting successful legal arguments against poll taxes, racial restrictions in housing and white primary elections.

In 1961, President John F. Kennedy selected Marshall for the U.S. Court of Appeals for the 2nd Circuit. The nomination initially was opposed by southern Democrats in the Senate, who claimed he lacked legal qualifications for the job. But Marshall was approved several months later, becoming the second black judge to sit on the 2nd Circuit.

Marshall served on the appeals court until 1965, when Johnson appointed him solicitor general of the United States, the government's top lawyer at the Supreme Court. Johnson had several civil rights victories at the court while Marshall was solicitor general, including high court approval for the 1965 Voting Rights Act.

Marshall also provided the government's backing to a case that led to the overturning of a California constitutional amendment prohibiting open housing legislation.

On June 13, 1967, at 11 a.m., Marshall called his wife, Cecilia, from the White House. "Take a deep breath and sit down slowly," he reportedly told her. Then Johnson's voice came on the line and told her Marshall had just been nominated to the Supreme Court.

The Senate confirmed Marshall 69 to 11 on Aug. 30, 1967, making him the first black justice in the court's 178-year history. He faced criticism from only a few southern senators, who attacked his "activist" temperament.

But Marshall was to join like-minded brethren. The court was then led by Chief Justice Earl Warren, who already had begun a judicial and social revolution.

Through the 1970s, Marshall was more regularly a steady vote for the opinions of liberal-leaning justices than author of major opinions himself.

In 1972, when the court struck down capital punishment as it was then being practiced, he wrote one of the most definitive statements on the death penalty:

"Death is irrevocable. Life imprisonment is not. Death, of course, makes rehabilitation impossible. Life imprisonment does not. In short, death has always been viewed as the ultimate sanction. . . . In striking down capital punishment, this court does not malign our system of government. On the contrary, it pays homage to it. . . . In recognizing the humanity of our fellow beings, we pay ourselves the highest tribute."

In that landmark ruling, Furman v. Georgia, the court set out procedural safeguards that states must follow if they wish to impose the death penalty, and since then a majority of the states have reinstituted capital punishment.

It was to be Marshall's dissents, particularly in death penalty cases, thundering with indignation, that gained most attention. He was suspicious of police searches and interrogation. He took a similar liberal tack in other areas, disdaining restrictions on speech, government expenditure benefiting religion and the weakening of environmental regulations.

In a partial concurrence in University of California Regents v. Bakke that endorsed a broader remedial use of race-conscious programs, he wrote in 1978: "It must be remembered that, during most of the past 200 years, the Constitution as interpreted by this court did not prohibit the most ingenious and persuasive forms of discrimination against the Negro. Now, when a state acts to remedy the effects of that legacy of discrimination, I cannot believe that this same Constitution stands as a barrier.

"At every point from birth to death, the impact of the past is reflected in the still-disfavored position of the Negro. In light of the sorry history of discrimination and its devastating impact on the lives of Negroes, bringing the Negro into the mainstream of American life should be a state interest of the highest order. To fail to do so is to ensure that America will forever remain a divided society."

Legal scholars say that Marshall's most important doctrinal contribution likely came in a dissent to the 1973 San Antonio Independent School District v. Rodriguez. In that Texas case a five-justice majority said an education is not a fundamental right guaranteed by the Constitution.

In an opinion by Lewis F. Powell Jr., the court said the constitutional guarantee of equal protection does not require that courts apply the strictest level of scrutiny to state decisions on how to finance public schools.

Marshall favored a different standard for determining whether state or federal laws violated equal protection guarantees, and his sliding scale approach influenced the court in later years to give greater scrutiny to government decisions and more broadly read equal protection guarantees.

In the years closer to his retirement, Marshall increasingly assumed a defensive role.

Until his close friend Brennan retired in 1990, it was just the two of them who would dissent from any decision that would lead to the execution of a defendant. He considered the death penalty immoral in principle and discriminatory in application.

"I'll never give up," he said in an interview in December 1983. "On something like that, you can't give up and you can't compromise. It's so morally correct."

On the day he resigned -- June 27, 1991 -- Marshall fired a parting shot that embodied his vigilance for criminal defendants and minorities generally.

It was in a dissent in Payne v. Tennessee, a case in which a narrow majority upheld the use of "victim impact" statements in death penalty cases, overruling two earlier cases that had prohibited such evidence from being introduced.

Marshall believed that the focus on a victim's character and his family's suffering would shift jury attention from whether the defendant was guilty to the victim's character and be difficult for the defendant to rebut.

Objecting to the conservative majority's overturning of precedent, Marshall wrote, "Tomorrow's victims may be minorities, women or the indigent. Inevitably, this campaign to resurrect yesterday's 'spirited dissents' will squander the authority and legitimacy of this Court as a protector of the powerless."

Marshall's overall health and his eyesight began to deteriorate in recent years. He had had a heart attack in 1976. He wrote fewer opinions and appeared to have difficulty reading from the bench the ones he did write.

He was hospitalized in 1987 with a blood clot in his right foot, and had been in and out of hospitals since.

But he never lost any of his exuberance.

Shortly before Marshall retired, Justice Byron R. White quipped to a law clerk, "In my 25 years here, Justice Marshall has told 1,000 stories and never the same one twice."

And friends say Marshall never forgot that he was black.

In his 1991 farewell news conference, he was asked whether he considered blacks, in the words of the Rev. Martin Luther King Jr., "free at last."

"Well, I'm not free. All I know is that years ago, when I was a youngster, a Pullman porter told me that he had been in every city in this country . . . and he had never been in any city in the United States where he had to put his hand up in front of his face to find out he was a Negro. I agree with him."

Marshall's first wife, Vivian Burney, died in February 1955. He married Cecilia A. Suyat in late December of that year. He is survived by his wife, Cecilia, and their two sons, Thurgood Marshall Jr. and John William Marshall, all of Northern Virginia, and four grandchildren.