Heathcote Williams at the Frankfurt Book Fair in 1980. He contributed to underground publications such as Heretic Visions and the "sex paper" Suck. (Richard Adams)

Heathcote Williams, a British playwright, poet, anarchist and magician who — in addition to founding a secessionist state with a group of London squatters — wrote one of the most acclaimed plays of the 1970s, along with best-selling poems about dolphins and whales, died July 1 at a hospital in Oxford, England. He was 75.

The cause was emphysema, said a daughter, Lily Williams.

Mr. Williams, a reedy Oxford University dropout who for many years sported black combat boots and a mass of curly red hair, emerged from Britain’s 1960s counterculture movement as a sort of artistic Prospero, a gifted but mischievous writer whose creative talents recalled those of Shakespeare’s sorcerer in “The Tempest.”

He wrote a dozen plays, many of them critical of society’s increasing obsession with celebrity; published several scholarly book-length poems on endangered animals; and co-founded an anarchist publishing house, Open Head Press, that skewered Britain’s royal family in pornographic postcards and scurrilous pamphlets.

Mr. Williams also appeared in more than a dozen film and television roles, including as Prospero in Derek Jarman’s 1979 adaptation of “The Tempest,” and helped start the “sex paper” Suck, an underground Amsterdam publication at the fore of Europe’s sexual liberation movement.

Mr. Williams in 1995. (Nils Jorgensen/Rex Features via AP)

He performed as a fire-breather (at one point accidentally setting himself on fire), practiced conjuring tricks, contributed to a television show about Charles Dickens’s love of magic, and struck up a relationship with Jean Shrimpton, the ’60s supermodel who helped popularize the miniskirt.

Despite being championed by figures ranging from the playwright and Nobel laureate Harold Pinter to Hollywood actor Al Pacino, Mr. Williams’s work often received little public attention — in large part because of its difficult subject matter and experimental style.

His groundbreaking play “AC/DC,” which premiered at London’s Royal Court Theatre in 1970 and opened in New York the following year, concluded with a trepanation — the piercing of a character’s skull.

The play, New York Times critic Charles Marowitz wrote in a review, placed Mr. Williams alongside Pinter, John Osborne and John Arden as one of the leading playwrights of the era. “It is,” he wrote, “the only play yet written to capture the tremulously combustible nature of the 21st century, which, because our mortal lives always trail chronology, is the century in which we are actually living.”

Performed amid closed-circuit television sets, with photographs of famous people plastered on the theater walls, the show was perhaps the first major theatrical work to criticize the glorification of celebrities in the television age, said New Yorker theater critic John Lahr.

“It’s arcane and not going to be popular, but as a little thought kit it puts all the others to shame,” he said in a phone interview. “His understanding of the imperialism of celebrity — no one comes close in the English theater. And that is one of the most toxic obsessions of our time.”

John Henley Heathcote Williams was born in Helsby, near Liverpool, on Nov. 15, 1941. His father was a lawyer, and his mother was a homemaker. Although Mr. Williams at times intimated that he had a working-class background, he graduated from Eton College boarding school and studied law at Oxford before dropping out about the time he published his first book, “The Speakers” (1964).

Written in a journalistic style, the work was a sympathetic portrait of the impassioned (and often drunk) orators of Speakers’ Corner in Hyde Park, where since the mid-19th century bums and activists have stood on soapboxes to hold forth on London’s social ills. “These are the only people I’d ever want to listen to,” Pinter wrote, reviewing the book for the London Observer. In a subsequent meeting at a barbershop, Pinter encouraged him to try his hand at playwriting.

The result was “The Local Stigmatic,” a one-act that premiered in 1966 and — featuring a pair of lowlifes who follow celebrities in the news and greyhounds at the track — presaged some of the themes of “AC/DC.” The show was a favorite of Pacino’s; he starred in its unsuccessful U.S. premiere and later produced and appeared in an hour-long film adaptation that was never given a theatrical release.

“When you read the ‘Stigmatic,’ you’re genuinely confused by it,” Pacino once told the writer Lawrence Grobel, acknowledging the play’s difficulty. “After two or three readings you start to see what he’s getting at, and it’s a revelation. It’s the difference between Heathcote Williams and other writers. Heathcote was writing about something that was deeply personal and much more far-reaching than the other plays of that period.”

Mr. Williams turned from playwriting to overt political activism by the late 1970s, joining with graphic designer Richard Adams to design and distribute pamphlets, postcards and other documents in the tradition of Britain’s 18th- and 19th-century “radical squibs.”

Their headquarters, in London’s Notting Hill neighborhood, eventually served as a base of operations for a short-lived secessionist state that Mr. Williams and other leaders called the Free and Independent Republic of Frestonia, as well as an organization called the Ruff Tuff Cream Puff Estate Agency, which helped squatters find abandoned buildings in London to use as temporary housing.

Survivors include his partner of more than five decades, Diana Senior of Oxford; their two daughters, China Williams and Lily Williams, both of London; a son, Charlie Gilmour of London, from a relationship with the novelist Polly Samson; a sister, Prue Cooper; and three grandchildren.

Mr. Williams’s commercial breakthrough was a departure from the skull-piercing of his early years. In “Whale Nation” (1988), “Falling for a Dolphin” (1989) and “Sacred Elephant” (1989), he mixed poetry with scholarship to spotlight endangered species that he sometimes studied firsthand, including on a six-month trip to the western coast of Ireland.

Still, politics was not a subject he could avoid for long. Last year he published a pamphlet, “The Blond Beast of Brexit: A Study in Depravity,” that collected unsavory quotes from Boris Johnson, who was then the mayor of London.

“There’s a German word for people like Johnson: Backpfeifengesicht,” Mr. Williams told the London Independent, explaining what drew him to the subject. “It means ‘a face that needs to be punched.’ ”