Frederic Rzewski, an American-born composer and pianist who worked in manifold musical genres throughout a career that spanned 60 years, died June 26 in Montiano, Italy. He was 83.

His publicist, Josephine Hemsing, confirmed the death, saying the cause was an apparent heart attack while dining with his daughter and grandson.

Mr. Rzewski (pronounced ZHEV-ski) was best known for a gigantic set of piano variations on a Chilean protest song — “The People United Will Never Be Defeated!” — which became an anthem of resistance after Gen. Augusto Pinochet overthrew the democratically elected socialist president Salvador Allende in 1973.

It is a sweetly sentimental tune, to which Mr. Rzewski added dissonances, slamming of the piano lid, quiet whistling from the player and near-impossible pianistic virtuosity that might have come from a mixture of Franz Liszt and the Niagara Falls.

The piece was created for pianist Ursula Oppens, Mr. Rzewski’s longtime friend, and received its first performance in 1976 as part of a series of concerts organized at the Kennedy Center in Washington to celebrate the U.S. Bicentennial.

“I wanted to write a piece that she could play for an audience of classical-music lovers who perhaps knew nothing at all of what was happening in Latin America,” Mr. Rzewski told the New York Times in 1997. “By virtue of listening to my piece for an hour, they might somehow get interested in the subject. I really was trying to reach the audience by using a language they would not find alienating.”

The work was an immediate success and has now been recorded many times. The pianist Igor Levit put it on a three-CD set with Bach’s Goldberg Variations and Beethoven’s Diabelli Variations — exalted company indeed. It won Gramophone magazine’s Recording of the Year award in 2016.

Reached on Saturday, Ms. Oppens reflected on the enormous popularity of the work and how “People” was perhaps the most celebrated virtuoso piano composition of recent years: “It’s become that mountain that pianists see and they feel they have to climb it — because it is there!”

Much of Mr. Rzewski’s work had a leftist political bent, but when a hapless reporter asked him if he was a Marxist composer, he replied, “Harpo or Groucho or what?”

More seriously, he addressed the issue with the Times in 1997, saying, “People keep harping on this political motif, and I’ve never understood why they think it’s so important. If it were pop music, it would be considered natural. But an American classical composer is supposed to be right-wing or an academic or just removed from reality.”

Another of his celebrated works was “Coming Together” (1973), a monodrama for actor and ensemble created after the uprising at the Attica state prison in New York.

“The design of ‘Coming Together’ is simple, even minimal,” the rock critic Robert Christgau wrote in the Village Voice. “Steve ben Israel reads and rereads one of [prisoner] Sam Melville’s letters from Attica over a jazzy, repetitious vamp. Yet the result is political art as expressive and accessible as ‘Guernica.’ In ben Israel’s interpretation, Melville’s prison years have made him both visionary and mad, and the torment of his incarceration is rendered more vivid by the nagging intensity of the music.”

Still, despite his occasional works for ensemble, Mr. Rzewski was best known for his compositions for solo piano. In 2003, Richard Dyer of the Boston Globe called him a thoroughly modern musician who also stood “in an honorable line of barnstorming pianist-composers that includes Chopin, Liszt, Prokofiev and Rachmaninoff. He is a daredevil pianist who has composed his most important works for his own instrument, and one of the three or four most significant composers for the piano in the past half-century.”

According to Mr. Rzewski, most of the reasons for his concentration on piano were pragmatic. “I tend to work with what is there,” he said in 2008. “Opera houses don’t come asking me to write operas. Symphony orchestras don’t come asking for symphonies. But there’s this piano player I see every day who keeps asking me for music. So that’s what I do.”

Frederic Anthony Rzewski was born in Westfield, Mass., on April 13, 1938, the son of two pharmacists. He began playing piano at the age of 3 and was composing soon thereafter.

He graduated in 1958 from Harvard University, where he studied with composers Randall Thompson and Walter Piston. He went on to earn earned a master’s degree in 1960 from Princeton University. His teachers there, Roger Sessions and Milton Babbitt, had recently turned the conservative campus into a center for the avant-garde.

Mr. Rzewski then went to Italy on a Fulbright Fellowship and worked with composer Luigi Dallapiccola in Florence. He performed and taught throughout Europe for most of the 1960s and played the first performances of Karlheinz Stockhausen’s “Klavierstück X” in 1962, later recording it for the German label Wergo.

In 1966 in Rome, Mr. Rzewski co-founded Musica Elettronica Viva, or MEV, with his fellow American composers Richard Teitelbaum and Alvin Curran. It was a little like an experimental rock band but with primitive electronic instruments rather than guitars. The group emphasized collective improvisation and audience participation, and their works often included dance and theatrical elements.

In 1977, Mr. Rzewski became a professor of composition at the Royal Conservatory in Liege, Belgium, a position he held until his death. He also taught master classes at many universities throughout Europe and the United States, including Yale, the University of Cincinnati, the University of California at San Diego and the California Institute of the Arts.

Oppens said Mr. Rzewski had been prolific during the pandemic. “He’s written eight or nine new pieces since the spring of 2020,” she said. One of his last pieces was called “Friendship,” and Oppens played its world premiere in March at the Mannes School of Music at the New School in New York.

Mr. Rzewski married Nicole Abbeloos in 1963, and together they had four children. They were never divorced. For about the past 20 years, his companion was Françoise Walot, with whom he had two children. Survivors include Walot, Abbeloos, his six children and five grandchildren.

Mr. Rzewski provided a fitting epitaph in 2003: “I wanted to find ways of writing that were chaotic, that bore some kind of relation to real subjective experience, and worked on various ways of writing down everything that was coming into my head. This was not so easy, because there are these mechanisms of censorship at work which tell you what to write, and what not to. Much of my work is incoherent, but systematically incoherent — a phenomenon that is familiar in literature and other artistic fields, but not in music. There were no surrealist composers.”

Tim Page won the Pulitzer Prize for criticism for his writings about music for The Washington Post. This fall, he will be a distinguished visiting professor at the Peabody Institute of Johns Hopkins University.

An earlier version of this story incorrectly stated that Igor Levit’s recording of music by Bach, Beethoven and Frederic Rzewski contained two CDs. It contains three CDs. The story has been corrected.