David Dellinger, whose commitment to nonviolent direct action against the federal government placed him at the forefront of American radical pacifism in the 20th century and led, most famously, to a courtroom in Chicago where he became a leading defendant in the raucous political conspiracy trial of the Chicago Seven, died Tuesday in a retirement home in Montpelier, Vt. He was 88.

His death was reported by Peggy Rocque, the administrator of the home, Heaton Woods.

An avuncular figure among younger and more flamboyant mavericks, Mr. Dellinger emerged in the 1960's as the leading organizer of huge antiwar demonstrations, including the encirclement of the Pentagon that was immortalized in Norman Mailer's account ''Armies of the Night.'' At the same time, making use of his close contacts with the North Vietnamese, he was able to organize the release of several American airmen held as prisoners and to escort them back from Hanoi.

In the often turbulent world of the American left, Mr. Dellinger occupied a position of almost stolid consistency. He belonged to no party, and insisted that American capitalism had provoked racism, imperial adventures and wars and should be resisted.

A child of patrician privilege, he had since his days at Yale learned and practiced strategies of civil disobedience in a variety of causes, steadfastly showing what he called his concern for ''the small, the variant, the unrepresented, the weak,'' categories he cited from the writings of William James.

In the federal courtroom in Chicago in 1969, when Judge Julius J. Hoffman presided over the trial of opponents of the Vietnam War charged with criminal conspiracy and inciting to riot at the Democratic National Convention a year earlier, Mr. Dellinger loomed over his co-defendants in age, experience, heft and gravitas.

The next oldest of the defendants, Abbie Hoffman, was 20 years his junior. Mr. Hoffman and Jerry Rubin were Yippies who mocked authority in star-spangled shirts; Mr. Dellinger favored quiet business suits. Tom Hayden, Rennie Davis, John R. Froines and Lee Weiner had led student movements; Mr. Dellinger had not.

Within this radical bouquet of representatives from what was called the New Left, Mr. Dellinger stood out as a link to a homegrown pacifist strain that had its roots within America's Old Left.

Paul Berman, who wrote about the radicals and revolutionaries who rose to prominence in the years around 1968 in ''Tale of Two Utopias,'' said that Mr. Dellinger ''came of age in one of the tiniest currents of the American left -- the Rev. A.J. Muste's movement for World War II pacifism, a movement based on radical Christian values and vaguely anarchist instincts. No rational person observing that movement during the 1940's would have predicted any success at all, and yet during the next two or three decades, Mr. Dellinger and his pacifist allies transformed whole areas of American life.''

Mr. Berman said that they ''did it by supplying crucial leadership in the civil rights revolution and by playing a central role in the mass movement against the war in Vietnam.''

''Dellinger, himself,'' Mr. Berman said, ''became the single most important leader of the national antiwar movement, at its height, from 1967 through the early 1970's. You could quarrel with some of his political judgments, but he was always sober, always resolute, always selfless and always brave.''

If his co-defendants in Chicago captured most of the attention of the news media, in the eyes of Judge Hoffman, it was Mr. Dellinger who had been the most guilty. The jury had acquitted all seven on conspiracy but found all but Mr. Weiner and Mr. Froines guilty of inciting to riot. Of the convicted, Mr. Dellinger was given the harshest penalty by Judge Hoffman, five years in jail and a $5,000 fine. He was also sentenced to two years and five months on the basis of 32 citations of criminal contempt for comments he made during the five-month trial, which ended in February 1970.

Two years later, with all the defendants free on bail, an appellate court, citing prejudicial conduct by Judge Hoffman, voided the convictions for inciting to riot. The next year another court upheld Mr. Dellinger's contempt conviction, but declined to impose sentence.

Mr. Dellinger was by his own lights more radical that many of his like-minded compatriots, a figure who often found the strategies and tactics of close colleagues like the Rev. Dr. Martin Luther King Jr. and Mr. Muste, his mentor in radical pacifism, too conciliatory.

When Mr. Dellinger was a young man, he had, for the experience, ridden boxcars with hobos during the Depression. Soon after, he drove an ambulance behind Loyalist lines in the Spanish Civil War and he traveled through Germany to witness the rise of Nazi power. He resisted the draft and in prison took part in hunger strikes to integrate the mess hall of Danbury prison. He edited Liberation magazine, which Mr. Mailer once described as ''an anarchist-pacifist magazine of worthy but not very readable articles in more or less vegetarian prose.''

In the 1940's Mr. Dellinger met Elizabeth Peterson at a Christian students' meeting, and they married. She survives him, as do two sisters, Nancy Marshall, of Massachusetts, and Elizabeth Cushman, of Sarasota, Fla. Also surviving are three sons, Patchen, of Seattle, Daniel, of St. Johnsbury, Vt., and Howard Douglas, of Nazareth, Pa.; two daughters, Natasha Singer, of Schnevus, N.Y., and Michele McDonough, of Croton-on-Hudson, N.Y.; nine grandchildren; and three great-grandchildren.

David Dellinger was born in Wakefield, Mass., on Aug. 22, 1915. His father, Raymond, was a lawyer and chairman of the town Republican Party, influential enough to take his son to a private White House lunch with Calvin Coolidge. His grandmother was active in the Daughters of the American Revolution.

Like his father he went to Yale, where he did well in his studies and was elected captain of the cross-country team. He also became close friends with Walt W. Rostow, who years later would face him from the other side of the barricades as a senior adviser on Vietnam to Presidents John F. Kennedy and Lyndon B. Johnson. He was drawn to pacifism through readings of Tolstoy and most particularly of ''The Power of Nonviolence,'' by Richard Gregg, an American who had spent years working with Mahandas K. Gandhi.

But his personal commitment to nonviolence came after a football game in which Georgia beat Yale. New Haven ''townies,'' who resented the Yale students, swarmed onto the field and tore down the goal posts. Mr. Dellinger joined in the melee and chased one young man, whom he hit and knocked unconscious. He recalled: ''I shall never forget the horror I felt the instant my fist struck solid flesh. When my victim fell, I dropped to my knees, lifted his head and I cradled him until he came to. I walked him home. I never saw my enemy again.''

He pledged he would never hit anyone again and forswore all violence.

In ''Armies of the Night,'' Mr. Mailer's account of the famous antiwar protest march on the Pentagon in 1967 that Mr. Dellinger helped to organize with Mr. Rubin, the writer compared the radical leader in nonviolent action to an alumni officer at a Yale reunion. ''He had the hard-working, modestly gregarious, and absolutely devoted sense of how mission and detail interlock, which is so necessary to good class agents, that rare vintage mixture of New England incorruptability and good fellowship.''

Mr. Dellinger graduated magna cum laude from Yale in 1936 with a degree in economics. He received a fellowship that enabled him to attend Oxford, where his interest in pacifism deepened. Returning to the United States, he entered Union Theological Seminary in New York, intent on becoming a minister, though he never got around to choosing a denomination. In 1940, as war clouds gathered, the United States required men to register for the draft. Mr. Dellinger and seven other seminarians announced that they would refuse to do so, despite assurances that as candidates for the ministry they would not be inducted into the Army. He wrote that accepting such a de facto exemption would in Gandhian terms amount to complicity with violence.

Mr. Dellinger, who was president of his class at the seminary, was expelled with the other dissenters and denounced from the pulpit by prominent churchmen across the country for what they contended was his doubtful patriotism. Mr. Dellinger had been living in a black neighborhood in Newark with several other draft resisters. Mr. Dellinger was tried for draft evasion, convicted and sent to Danbury prison for a year. In 1943, with America at war, Mr. Dellinger returned to Newark and was again summoned to report for a preinduction physical examination. Again he refused to report. He was arrested and convicted of draft evasion and sent to Lewisburg, a maximum security prison, for two years.

In 1956 along with Mr. Muste and Dorothy Day, the Catholic anarchist, he founded Liberation, eventually becoming its editor and and publisher. It was the escalating war in Vietnam that brought Mr. Dellinger into heightened prominence. In 1965, he helped to sponsor the first major antiwar demonstration in New York, which took place in October, involving liberal and radical groups. Mr. Dellinger, along with a few other opponents of the war, sent invitations to antiwar groups to join in another protest against the war during the Democratic National Convention in Chicago in August 1968, focusing on what was anticipated as the renomination of President Johnson. Although Johnson declined to run again, groups massed in Chicago, where violent clashes with police led to the indictments that led to the indictments of the Chicago Seven and Bobby Seale of the Black Panthers. For a while it was the Chicago 8, but Judge Hoffman had Mr. Seale removed from the trial and the courtroom after ordering him bound and gagged after he insisted that he was being denied his right to a lawyer of his own choice.

Even before the trial ended, Mr. Dellinger, who had earlier established close contacts in North Vietnam, flew to Hanoi in August 1969 to escort home three American servicemen who had been held as prisoners. He made a similar trip in 1972, and during much of the war he served as a conduit to North Vietnam, advising the Hanoi government which Americans should be permitted to visit there and making travel and visa arrangements. During the peace talks in Paris he was a consultant to the North Vietnamese delegation.

In the 1970's, Mr. Dellinger and his family moved to a house on a dirt road in Peacham, Vt., where he made what he described as a precarious living teaching in the adult education program at Vermont College in Burlington and writing. In addition to his autobiography, he wrote ''Revolutionary Nonviolence'' (Anchor Books, 1971), ''More Power Than We Know'' (Anchor Books, 1975) and ''Vietnam Revisited'' (South End Press, 1986).

The dedication of this last one read, ''To all veterans of the Vietnam War; those who fought in it and those who fought against it.''