Maria Teresa Horta, a Portuguese feminist writer who helped shatter her conservative country’s strictures on women, died on Feb. 4 at her home in Lisbon. She was 87.

Her death was announced on Facebook by her publisher, Dom Quixote. The Portuguese prime minister, Luis Montenegro, paid tribute to her on X, calling her “an important example of freedom and the struggle to recognize the place of women.”

Ms. Horta was the last surviving member of the celebrated writers known as the “Three Marias,” who together wrote the landmark 1972 book “Novas Cartas Portuguesas” (“New Portuguese Letters”). A collection of letters the women wrote to one another about their problems as women in Portugal, it opened up a world of repressed female sexuality, infuriated the country’s ham-fisted dictatorship and led to their arrest and criminal prosecution on charges of indecency and abuse of freedom of the press.

“To feminists around the world, as well as to champions of a free press, the police action against the Portuguese women in June 1972 was an outrage that slowly became the focus of an international protest movement,” Time magazine wrote in July 1973.

The Three Marias — Ms. Horta, Maria Isabel Barreno (1939-2016) and Maria Velho da Costa (1938-2020) — became international feminist folk heroes, and the book’s fame alerted the world to repression under the Portuguese dictatorship. Simone de Beauvoir, Marguerite Duras and Adrienne Rich were among the writers who declared their public support. The National Organization for Women voted to make the case its first international feminist cause.

The case was not Ms. Horta’s first brush with controversy.

In 1967 she had been “beaten in the street” after the publication of her breakthrough volume of poetry, “Minha Senhora de Mim” (“My Lady of Me”), she told her biographer Patrícia Reis in 2019. That book “challenged something deeply rooted in this country,” she said: “the silencing of female sexuality.”

Frequent knocks on the door by the Portuguese secret police became part of her life.

The themes of her work grew from what she characterized as a dual oppression: being a woman in Portugal’s male-dominated society and growing up in a police state.

“I was born in a fascist country, a country that stole liberty, a country of cruelty, prisons, torture,” she told an Italian interviewer in 2018. “And I understood very early on that I couldn’t stand for this.”

She also wouldn’t stand for the oppression of women in Portugal’s traditional macho culture. “Women are beaten or raped just as much by a doctor, a lawyer, a politician, whoever, as by a worker, a peasant and so on,” she told the Lisbon daily Diário de Notícias in 2017. “Women have always been beaten and have always been raped. People do not consider the violence that goes on in bed, in the sexual act with their husband.”

In 1971, these preoccupations inspired Ms. Horta to start meeting every week with two friends and fellow authors, Ms. Barreno and Ms. da Costa, to share written reflections on the common themes that troubled them.

They were inspired by a classic work from the 17th century, “Letters of a Portuguese Nun,” supposedly written by a young woman shut up in a Portuguese convent to the French cavalry officer who had abandoned her. Scholars now believe the work was fiction, but its powerful expression of pent-up longing and frustration resonated with the three Marias.

Like the nun in the book, they used letters to one another, as well as poems, to express their unhappiness as women in their early 30s, educated by nuns, married and with children, in a Lisbon stifling under a 35-year dictatorship, rigid Catholicism and ill-judged colonial wars in Africa.

When they published the writings as “New Portuguese Letters,” they vowed never to reveal to outsiders, much less the police, who had written what.

“Their views and natures were far apart,” Neal Ascherson wrote in The New York Review of Books in a review of the 1975 English translation, titled “The Three Marias.” “Maria Isabel the coolest, Maria Teresa the gaudiest personality, Maria Fátima the one who swerved away from pure feminism toward social and psychological analyses of a whole people’s oppression.”

The strange hybrid — Mr. Ascherson called it “a huge and complicated garland” — is suffused with repressed rage at the condition the women find themselves in.

“They wanted the three of us to sit in parlors, patiently embroidering our days with the many silences, the many soft words and gestures that custom dictates,” one of the letters says. “But whether it be here or in Beja, we have refused to be cloistered, we are quietly, or brazenly, stripping ourselves of our habits all of a sudden.”

Another letter says, “We have also won the right to choose vengeance, since vengeance is part of love, and love is a right long since granted us in practice: practicing love with our thighs, our long legs that expertly fulfill the exercise expected of them.”

Although Mr. Ascherson found the book “often maddeningly imprecise, self-indulgent and flatulent,” he said that “where it is precise, the book still bites” and “where it is erotic, it is neither exhibitionist nor coy but well calculated to touch the mind through emotion.”

A few Portuguese reviewers welcomed it as “brave, daring and violent,” as the author Nuno de Sampayo put it in the Lisbon newspaper A Capital. They predicted a difficult reception.

Prime Minister Marcello Caetano attempted to put the authors in jail, calling them “women who shame the country, who are unpatriotic.”

On May 25, 1972, the state press censor banned the book. The next day it was sent to the criminal police department in Lisbon. When the authors’ trial opened in 1973, the crowd was so great that the judge ordered the courtroom cleared.

In May 1974, nearly two years after their arrests and two weeks after the Portuguese dictatorship was overthrown, the Three Marias were acquitted.

Judge Artur Lopes Cardoso, who had been overseeing the case, became a sudden convert, declaring the book “neither pornographic nor immoral.” “On the contrary,” he said, “it is a work of art of high level, following other works of art produced by the same authors.”

Maria Teresa de Mascarenhas Horta Barros was born in Lisbon on May 20, 1937, the daughter of Jorge Augusto da Silva Horta, a prominent doctor and a conservative who supported the dictatorship, and Carlota Maria Mascarenhas. Her paternal grandmother had been prominent in the Portuguese suffragist movement.

Maria attended Filipa de Lencastre High School, graduated from the Faculty of Arts at the University of Lisbon, and published her first book of poetry at 23. She would go on to write nearly 30 more, as well as 10 novels.

She was also a critic and reporter for several newspapers and the literary editor of A Capital.

In the 1980s, she edited the feminist magazine Mulheres, which was linked to the Portuguese Communist Party. (She was a member of the party from 1975 to 1989.)

No matter the genre — poetry, fiction or journalism — she considered writing a public duty.

“The obligation of a poet is not to be in an ivory tower; it is not to be isolated but to be among people,” she told the online magazine Guernica in 2014. “As a journalist, I never isolated myself. I was a journalist at a daily newspaper and every day I went out on the street. Every day I had contact with people.”

She won most of her country’s top literary prizes, but she caused a stir in 2012 when she refused to accept the D. Dinis Award because she objected to the government’s right-leaning politics.

She is survived by her son, Luis Jorge Horta de Barros, and two grandsons. Her husband, the journalist Luis de Barros, a former editor of the newspaper O Diário, died in 2019.

“People ask me why I am a feminist,” Ms. Horta told Guernica in 2014. “Because I am a woman of freedom and equality and it is not possible to have freedom in the world when half of humanity has no rights.”

Kirsten Noyes and Daphné Anglès contributed research.