The recent death of Rosa Parks refocused national attention on one of the most beloved figures of the civil rights movement. But without the heroism of thousands of unsung grass-roots activists, the movement would never have accomplished what it did. In "Freedom Riders," Raymond Arsenault, a professor of history at the University of South Florida, rescues from obscurity the men and women who, at great personal risk, rode public buses into the South in order to challenge segregation in interstate travel. Drawing on personal papers, F.B.I. files and interviews with more than 200 participants in the rides, Arsenault brings vividly to life a defining moment in modern American history.

"Freedom Riders" begins not on May 4, 1961, when 13 black and white volunteers boarded two buses in Washington bound for New Orleans, but 17 years earlier, when Irene Morgan, in an act of defiance that anticipated Rosa Parks's, refused to give up her seat on a bus traveling from Virginia to Maryland. Convicted of violating local segregation laws, she appealed all the way to the Supreme Court, which ruled in 1946 that segregated seating on interstate buses violated the Constitution.

In 1947, the Congress of Racial Equality, or CORE, an obscure civil rights group founded a few years earlier by Christian pacifists, organized the Journey of Reconciliation to test compliance. The trip through the upper South went off peacefully, but failed to dent the edifice of segregation. As Arsenault notes, there had been Reconstruction-era battles over integrating streetcars and railroad carriages, and late-19th-century lawsuits brought by black travelers demanding equal treatment. The Freedom Rides of 1961, also organized by CORE, represented the latest front in a battle that had begun decades before.

In most parts of the world, a bus journey would hardly have attracted attention. In the Jim Crow South of 1961, the Freedom Riders encountered shocking violence that deeply embarrassed the Kennedy administration. Outside Anniston, Ala., a mob set one of the buses on fire. The riders were lucky to escape with their lives. In Birmingham, police officers gave Klan members 15 minutes to assault the riders at the bus station before intervening. The result was what Arsenault calls "one of the bloodiest afternoons in Birmingham's history."

Further violence followed another group of riders in Montgomery, where John Seigenthaler, the president's personal representative, suffered a fractured skull and several broken ribs. It took a small army of policemen and National Guard troops to escort the bus from Montgomery to Jackson, Miss., where the Freedom Riders were promptly arrested for breach of the peace and attempting to incite a riot. Some spent time at the infamous Parchman Farm, a prison plantation the historian David Oshinsky called "synonymous . . . with brutality."

Arsenault relates the story of the first Freedom Ride and the more than 60 that followed in dramatic, often moving detail. He reminds us of the personal courage and organizational ability of forgotten catalysts of the movement like Diane Nash, a black student leader in Nashville who helped to mobilize new groups of Freedom Riders upon hearing of the first beatings.

As its title suggests, the book focuses above all on the riders themselves. Future scholars will be grateful for the appendix, which provides brief biographical information on more than 400 of them. Unfortunately, apart from a table showing that they were overwhelmingly young (three-quarters were under 30), mostly male and almost equally divided between black and white, this information remains unanalyzed. "Diversity," Arsenault writes, "was the hallmark of the Freedom Rides." The first group illustrates his point — it included a Wall Street stockbroker, a veteran unionist from Michigan, a folk singer from New York, a former Navy captain and a Nashville theology student (John Lewis, now a member of Congress). But "diversity" is a description, not an interpretation. One wishes for a more detailed account of the riders' political backgrounds, organizational connections and later experiences.

As Arsenault makes clear, the Freedom Rides revealed the pathology of the South. This was a society not simply of violent mobs but of judges who flagrantly disregarded the Constitution, police officers who conspired with criminals and doctors who refused to treat the injured. Southern newspapers almost universally condemned the riders as "hate mongers" and outside agitators (even though about half had been born and raised in the South). Not that the national press acquitted itself much better. The New York Times reporter Claude Sitton produced some of the best coverage in the country. But the paper's editorials, while defending the right to travel, called on the riders to halt their "courageous . . . but nonetheless provocative action."

Most remarkable was the supine response of the Kennedy administration. Before assuming the presidency, John F. Kennedy had evinced little interest in civil rights. Once in the White House, he viewed the Freedom Rides as an unwelcome distraction from his main concern — the cold war. Kennedy's first impulse was to try to keep details of the violence out of the press. In the midst of the crisis, he delivered a special address to Congress. Remembered today for its pledge to put a man on the moon, it dealt primarily with international affairs, identifying the "southern half of the globe" as "the great battleground for the expansion of freedom today." It made no mention of the battle for freedom then being fought in the southern United States.

The attorney general, Robert F. Kennedy, comes off rather better. Initially as impatient with the riders as his brother, Robert Kennedy became emotionally committed to their cause. It was he who petitioned the Interstate Commerce Commission to ban segregation in interstate bus travel. The result was an order that brought the Freedom Rides to a triumphant end.

Overall, the administration's response calls into question a staple of recent writing on the civil rights movement — that the cold war created a favorable context for racial change. Certainly, the photographs that flashed across the world embarrassed the White House. But the conflict with the Soviets also inspired deep distrust of any movement that included critics of American foreign policy. After a telephone conversation in which he urged Martin Luther King Jr. to restrain the riders, Robert Kennedy remarked to an aide, "I wonder whether they have the best interest of their country at heart."

The cold war did not produce a significant change in federal policy. That, as both the Freedom Riders and King knew, required a social movement. Indeed, of the civil rights leaders touched on in this book, King comes across as the most supportive of the young activists. We sometimes forget how young King himself was at this time. Only 32 in 1961, he was closer in age to the riders than to the older civil rights establishment. In his conversation with Robert Kennedy, King refused to heed an appeal for moderation: "I am different from my father. I feel the need of being free now." This impatience for freedom, acted out by the courageous young Freedom Riders, helped propel a reluctant America at least part of the way down the road to racial justice.