Jacqueline Bouvier Kennedy Onassis, 64, who died last night, became an icon of American style when her husband was president and the closest thing Americans had to royalty after his assassination.

In the 35 months that John Fitzgerald Kennedy was president, his young and beautiful wife captivated the world with her understated elegance, her whispery voice, her picture-perfect children and her commitment to culture. But the picture forever burned into the country's memory was of the woman in a blood-stained pink suit standing beside Lyndon Baines Johnson as he took the oath of office in November 1963. Even in this moment of terrible sorrow she maintained her dignity and composure.

Five years later, she shocked the world by marrying Greek shipping tycoon Aristotle Onassis, 22 years her senior, and light-years from her aristocratic upbringing.

When he died in 1975, leaving her a wealthy woman, she entered the work force as a $200-a-week associate editor for Viking Press. Two years later, angry when Viking bought a Jeffrey Archer novel that involved the attempted assassination of her brother-in-law, Sen. Edward M. Kennedy, she quit and moved to Doubleday.

In recent years, her constant companion was diamond merchant Maurice Tempelsman. She retained her girlhood love of horses and was a regular with Middleburg's ultra-exclusive Orange County Hunt, the only non-property owner welcome to ride.

She was only 31 when her husband became president. Though well-educated, well-read, fluent in French and comfortable in Spanish, she had held no substantive job before her marriage. (She had been an inquiring photographer for the Washington Times-Herald in the early 1950s.) She entered the White House as a traditional First Lady and dutifully walked two paces behind her husband on occasions of state.

Although her life was much-chronicled and her later days were dogged by paparazzi photographers, she carried the secrets of her private life to her grave. She never spoke or wrote about her personal relationships, even after Jack Kennedy's philandering had become widely known. To the public, she and Kennedy had appeared to be the world's most glamorous couple.

As befit a woman who came of age in the 1940s, she never intruded in politics or policy and in the early years of her marriage seemed to have no interest in it.

Yet Robert McNamara, who was Kennedy's secretary of defense, said yesterday that Jacqueline Kennedy had importuned him to stop the Vietnam War in 1964, a year after she left the White House.

"She had a great sense of compassion," he said. "She was horrified by violence, particularly violence done in war, and particularly the Vietnam War. I recall her pleading with me to stop the violence. She literally beat on me physically with her fist once when I was having dinner with her in New York. She was saying, 'This killing must stop!' "

Growing Up Privileged

She was born on July 28, 1929, at Southhampton, N.Y., to New York stockbroker John "Black Jack" Bouvier and Janet Lee, a couple of wealth and privilege who moved in the upper reaches of New York society. When she was 8 years old her parents separated, which proved a devastating experience for the child, who adored her father. Later, Janet Bouvier married the wealthy Washington lawyer and stockbroker, Hugh D. Auchincloss, and Jackie's world revolved around Merrywood, the Auchincloss estate in McLean, and their Newport, R.I. home, Hammersmith.

She made her debut when she was 17 and studied at Miss Porter's in Farmington, Conn.; Vassar College; the Sorbonne; and finally George Washington University. She was a talented writer who won Vogue magazine's Prix de Paris contest, which earned her a stint at its Paris office. Back in Washington, she met a young Massachusetts congressman, John F. Kennedy, through a mutual friend, Washington journalist Charles Bartlett.

In 1953, her friendship with Kennedy, by then a U.S. senator, turned serious. They were married on Sept. 12 at Hammersmith Farm before 1,700 guests. She had been a Republican before she and Kennedy met, and even afterward was a reluctant participant in his political endeavors. By the time Kennedy won the presidency, she was resigned to what lay ahead. "I'd be a wife and mother first, then First Lady," she told the press.

White House Work

When she arrived in the White House, she turned her attention to its restoration with an organizational talent that surprised even her husband. Hugh Sidey, then White House correspondent for Time, said Kennedy called him at home one day to say that his wife had an idea she wanted to try out on him.

"She said she wanted to restore the White House to the style of Jefferson and Madison," he recalled yesterday. "She wanted to write a piece for Life and asked how much we paid because she wanted to get a fund going. When we finally got the piece, it was about 12 feet long rolled up and clipped together. She'd written it on yellow legal paper. We paid about $50,000 for it. But it was worth it."

Clark Clifford, who once served as Kennedy's attorney, said she was unrelenting and extremely persuasive when she found a painting or a piece of furniture she wanted for the White House. "One day she decided the White House should have a picture of Benjamin Franklin," Clifford said. "There were only two authentic portraits of him and only one of them for sale."

That painting belonged to millionaire Philadelphia publisher Walter Annenberg, who had refused to contribute to JFK's campaign, but she called him anyway. He had paid $500,000 for the work.

"I told her it was an interesting thought but that I wanted to think about it a few days," Annenberg recalled yesterday. "But I called her back within an hour and told her that as much as I loved that painting, it was more appropriate that it be in the White House than in my home."

In addition to replacing the White House's shabby gentility with professional interior design, she was determined to raise the arts in America to a level of great stature. She hoped that Washington would become a world cultural center.

"She epitomized the flowering of a new generation that was taking over America," said Benjamin C. Bradlee, former executive editor of The Washington Post, who was close to the First Couple during Kennedy's presidency. "She lit up the White House and lit up the land with a sophisticated charm that hadn't been seen in the White House."

The Kennedys' dinners were dazzling affairs, starring such cultural giants as cellist Pablo Casals, poet Carl Sandburg, violinist Isaac Stern and composer Igor Stravinsky.

In later years, too, she remained a firm supporter of the arts. When American Ballet Theatre was teetering on the edge of bankruptcy last year, Onassis spearheaded a fund-raising drive that gave ABT an influx of $3 million.

The Jackie Look

Her style was unique, and widely copied. A rare combination of American simplicity and jet-set good taste, it seemed to sum up the early '60s. The pillbox hats, the sleeveless dresses, the long white gloves, the boxy jackets with three-quarter-length sleeves: These were the elements of the "Jackie Look."

When it became clear that she dare not wear European clothes as First Lady, Mrs. Kennedy turned to the Paris-born American designer Oleg Cassini. In his 1987 autobiography, "In My Own Fashion," Cassini related that when she asked him to be her designer, he agreed, provided he was the only one.

"Can you do it by yourself, though?" she asked. "I'm going to need an awful lot of clothes."

In later years, after she had married Onassis, her style was characterized by low-key luxury. "By that time," said designer Bill Blass, "she had truly developed a style that was her own -- a kerchief on her head, a black T-shirt, white pants and the dark glasses. Everyone in her class dressed that way. But she personified it for nearly three decades."

Her concern for her children's privacy dominated her later years. Friends remembered how she shielded them from the glare of publicity. "She was always concerned about what it would do to them," said Clark Clifford, who was Kennedy's personal attorney and had assisted Mrs. Kennedy in her White House fund-raising efforts.

"She was determined that they would be allowed to develop into human beings apart from the paparazzi -- and they did," said McNamara.

Of her four children, only Caroline and John Jr. survived. Patrick, the baby born while Kennedy was president, died shortly after birth. Earlier, another baby was stillborn. Mrs. Kennedy also experienced a miscarriage. Caroline Kennedy Schlossberg is president of the John F. Kennedy Presidential Library in Cambridge, Mass., and the mother of three children: Rose, Tatiana and John, whom the family calls Jack. John Kennedy Jr. is a New York lawyer.

On Her Own

At the midtown offices of Doubleday, where Onassis had quietly carved out a solid career in publishing, "people here today are just devastated," said president Stephen Rubin. For 16 years she'd arrived by cab each Monday, Tuesday and Wednesday morning for the normal round of meetings, discussions with authors and agents and consultations with the art and promotion departments.

Among the books she acquired, edited and shepherded -- about a dozen each year -- were three Bill Moyers bestsellers, memoirs by dancers Judith Jamison, Martha Graham and Gelsey Kirkland, Michael Jackson's "Moonwalk" and the "Cairo trilogy" of novels by Egyptian Nobel laureate Naguib Mahfouz.

"She really liked to be treated like one of the guys," said a former Doubleday associate. "She wanted people to call her Jackie. She had a very small office, smaller than others."

In a rare interview with Publishers Weekly last year, Onassis said, "One of the things I like about publishing is that you don't promote the editor -- you promote the book and the author."

Onassis took the same approach to promoting another passion: historic preservation. On the board of the Municipal Art Society for close to 20 years, she was credited with helping to save Grand Central Station from destruction and blocking a skyscraper that would have thrown shadows across a swath of Central Park.

"You'd plan a rally and if she was going to be there, there'd be 5,000 people there. If not, you'd be lucky to rustle up 500," said preservationist Frederic Papert.

He said he knew she disliked the attention -- "she was a private and, I think, shy person" -- but admired her willingness to barter her visibility for attention to city treasures. "I think it's reasonable to say that if you care about landmarks and preservations, you owe a big debt to her."

Staff writers Sarah Booth Conroy, Lloyd Grove, Cathy Horyn, Paula Span, David Streitfeld and Jacqueline Trescott contributed to this report.