Led by a bearded flutist, in a buoyant atmosphere of balloons, banners and anti-nuclear chants, an estimated 550,000 protesters gathered here today in the nation's largest disarmament rally and one of the biggest demonstrations in U.S. history.

Starting from the United Nations and walking in two enormous parades up Fifth and Seventh avenues to the rally in Central Park, the protesters' march was a peaceful, festive family affair. There were no reported arrests or violent incidents.

Created in the wake of the burgeoning European anti-nuclear movement, the protest was organized by an umbrella group, the June 12 Rally Committee, "to support the United Nations Special Session on Disarmament and to call for a freeze and reduction of all nuclear weapons and a transfer of military budgets to human needs."

But the stated purpose clearly meant different things to different people. Unlike the Vietnam-era peace movement, which sought to end the war in Southeast Asia, this rally brought together groups with widely different proposals for slowing the nuclear arms race.

Yet all seemed united in their fear of nuclear holocaust.

Marching today were groups from Hiroshima, Japan, protesters from Europe, Gray Panthers from Washington, D.C., community groups from across the Midwest, familiar anti-war crusaders--such as Joan Baez, David Dellinger and former representative Robert Drinan.

There were veteran activists like Lord Philip Noel Baker, the 92-year-old cochairman of World Disarmament and a Nobel Peace Prize winner, who--as he was pushed in a wheelchair--recalled that he had spent World War I as an ambulance driver, a conscientious objector even then.

And there were ordinary people like Lillian Wexler, a New Yorker who cheered the marchers on with a "Kill The Bomb, Save The People" sign she had written on the back of a yellow plastic placemat.

"I've got eight grandchildren and two great-grandchildren," she said, as the parade passed by on Fifth Avenue. "Those are my reasons for being out here today."

"After 37 years, America and the world have come to understand that nuclear bombs have to be banned," Drinan said, citing the dropping of atomic bombs on Japan at the close of World War II.

"This world has never known a single moment of such deadly jeopardy," warned actor Orson Welles, who was joined at the rally by New York Mayor Ed Koch, Bella Abzug, the Rev. William Sloane Coffin Jr. and entertainment celebrities such as film stars Jill Clayburgh, Susan Sarandon and Roy Scheider.

Six months in the planning by groups ranging from The American Friends Service Committee to the Southern Christian Leadership Conference, the massive rally cost the funding groups an estimated $600,000; one-third raised from rock concerts, the remainder from General Motors heir Stewart Mott, members of the Rockefeller family, and philanthropic organizations.

New York City paid an estimated $1.25 million for police and other city services, putting 6,000 police in the street for the demonstration. They were used, however, mostly for traffic control.

Today's crowd estimate of 550,000, reached by police by measuring the Great Lawn, drawing a grid and counting people within, eclipsed totals for many previous gatherings.

The largest previous gathering recorded in Central Park was 400,000 who heard Simon and Garfunkel on the Great Lawn last fall; about the same number attended the 1969 Woodstock, N.Y., rock music festival.

Demonstrations in Washington against the Vietnam war in November, 1969, and in April, 1971, and civil rights protests in 1963 drew crowds estimated at about 200,000 by police and much higher by organizers. More than 1 million people attended the Mass celebrated Oct. 5, 1979, by Pope John Paul II in Chicago's Grant Park.

At sunrise today, marchers began to gather outside the United Nations. The feeling was international, purposeful, and somewhat spiritual. There were talks by religious leaders and diplomats. "We as men, women and children want to survive--so let the arsenal be destroyed," said Oluyemi Adeniji, Nigerian ambassador to the United Nations.

By 10:30 a.m., behind a red banner reading "Freeze The Arms Race," the group was heading up Fifth Avenue. At the fore was flutist Larry Qubeck from Kentucky, a subsistence farmer who had hitched to New York for the rally with $35 in his jeans and whose first memories of the nuclear threat, he said, were at age 3, watching his father build a fallout shelter.

Behind him came the multitudes: the Bread and Puppet Theater in white masks; families pushing infants in strollers; community groups from the middle and far western states.

"Glow Spiritually, Not Radioactively," read the buttons. "End the Arms Race, Not the Human Race," "Make Love, Not Nuclear War."

In front of St. Patrick's Cathedral, at Fifth Avenue and 50th Street, the festive atmosphere received a little extra perk--a bride and groom. Dr. Kevin and Paula Maloney, married about five minutes, saw the oncoming parade as they left the church and stepped out, briefly, to join it. They stopped the march. They kissed in front of it for the photographers. They kissed again. Did the two support the cause, he was asked. "We sure do," he said.

Orderly despite its size, the march lacked the bitterness of the last days of the anti-Vietnam protests, at times taking on the gaiety of a crafts fair or even a carnival midway. In front of the Dakota apartment building, where John Lennon lived and was murdered, dealers did a brisk business in buttons and '60s "collectibles."

Inside Central Park, en route to the Great Lawn, where protesters lay down in a picnic-like atmosphere and listened to the music and speakers, a golden retriever wore a sign that said "Dogs for Disarmament." As in a midway, and for a quarter a shot, the protesters could slam an aluminum bomb with a sledgehammer.

"It's hard to label this crowd," '60s activist Abbie Hoffman, now of upstate New York, said. "I'm down here with 10 percent of my town--that's nine people, in a town where maybe two are Democrats, two are independents and the rest are Republicans. I would say a lot of the people here are grassroots Americans. I would bet a lot of them have never been to a demonstration before."

Organizers, many leaders of earlier peace movements and sympathetic political figures attending the rally insisted that the disarmament movement and rally involved a different constituency than those of a decade ago.

In an interview, Rep. Toby Moffett (D-Conn.), who marched as a part of the earlier anti-war movement, called the demonstration "overpowering . . . the most important movement I have ever been associated with. This is family based."

Another young Democrat, Massachusetts Rep. Edward Markey, called the rally part of a "middle-class movement. These are people with real clout in their communities," Markey said, who along with Moffett and Reps. Thomas J. Downey (D-N.Y.) and Mickey Leland (D-Tex.) appeared at the demonstration.

Although Leland, a black, read a statement backing the freeze from the Congressional Black Caucus, the marchers were largely white. Leland said black activists are "focused on the economy" and that the anti-nuclear movement will "continue to be a very white group" as long as the poor are fighting Reagan administration budget cuts.

"Peace and disarmament are important to blacks but economics in general are much more pressing," said William Lucy, president of the Black Trade Unionists, who endorsed the freeze.

There were barbs directed at administration budget cuts and defense policies, and President Reagan was on the minds of the protesters and the political figures here.

Moffett and Downey said they would attempt to use the turnout today as a selling point for an effort to overturn administration defense budget proposals nearing House action.

But Defense Secretary Caspar W. Weinberger, in an interview taped Friday for broadcast tonight on WNET-TV in New York, said: "The fact that a very large number of people turn out for a particular event is certainly something that people notice.

"But I don't think that anybody rushes back and says, 'We have to change our policy' . . . or something because there's been a rally."