A spokesman for the Campaign for Nuclear Disarmament, Pádraig McCarrick, confirmed the death but did not specify a cause.
For more than six decades, Ms. Arrowsmith was among the most prominent anti-nuclear campaigners in Britain — including as part of the civil disobedience group Committee of 100, co-founded by Nobel laureate Bertrand Russell — and challenged the British and U.S. military around the world.
There were some victories. She led an effort that pushed the British armed forces to publicly acknowledge in the early 1980s that service members would not be automatically disciplined for joining anti-nuclear movements. Mostly, however, Ms. Arrowsmith aligned with grand-gesture protests that captured headlines and became symbolic rallying points for government opposition.
For years, Ms. Arrowsmith passed out leaflets calling for British military personnel to reject deployment to Northern Ireland. During the Vietnam War, she joined peace activists on the South Vietnam-Cambodian border in 1968 seeking to deter U.S. bombing.
In 1990, she camped with others in the Iraqi desert near the Saudi border to protest the looming U.S.-led invasion that pushed Iraqi occupation forces from Kuwait — fearing the battles could have touched off a wider regional war.
She was jailed almost a dozen times in Britain — for convictions that included sit-ins and violating national security rules by urging personnel to defect — and waged several hunger strikes while behind bars. In 1961, she was force fed after refusing food to protest that canvas bags sewn in a prison workshop could be used as sandbags in combat. The incident was raised in Parliament.
In 1974, during another sentence, she walked away from a campus-style penitentiary and then spoke at a left-wing demonstration in London’s Hyde Park. “I shouldn’t have been in prison at all,” she told the Guardian in 2008. “I feel guilty for not trying to escape from all my prison sentences.”
Still, she always added doses of realism when interviewers discussed her reputation as a tireless resister. Nuclear weapons remain, she reminded them, and the technology for warheads has expanded to countries such as North Korea.
“During the Cold War, we lived on the edge of nuclear war by accident,” she said. “I don’t feel we are on the edge now, although the proliferation of nuclear weapons among smaller countries has not made the world a safer place.”
“I suppose,” she added in a 2015 oral history, “I’m rather pessimistic, you know.”
Ms. Arrowsmith recalled how she was at first uplifted when she heard about the U.S. atomic bombing of Hiroshima and Nagasaki to end the war in the Pacific. As a teenager at an elite boarding school, Cheltenham Ladies College, she assumed the nuclear blasts were just far bigger versions of the German bombs dropped on Britain, including one that hit the Arrowsmith family garden in Torquay while they were away. She soon became terrified of the new weapon and the idea that any incident now had the potential to escalate into a nuclear nightmare.
At the University of Cambridge in the late 1940s she took part in antiwar rallies and then, on a Fulbright scholarship to Ohio University, she joined pacifist and civil rights groups led by the Quakers and others. “We were all aware a third world war would involve, you know, probably complete nuclear annihilation,” she said.
After returning to Britain, Ms. Arrowsmith became riveted by stories about an anti-nuclear activist who planned to sail into the Pacific to try to stop Britain’s first nuclear weapon test in 1957. The voyage never took place, but Ms. Arrowsmith made contact with other anti-nuclear groups in Britain. A proposal was made to trek from London’s Trafalgar Square to the country’s nuclear weapons research site in Aldermaston, about 50 miles away.
Ms. Arrowsmith was dismissed from as a part-time nursing assistant at a psychiatric hospital for handing out leaflets promoting the protest march. She became a full-time organizer for the protest event staged in April 1958.
“We were expecting 50 people,” she recalled. “We got 8,000.”
The march was a defining moment for Britain as one of the first major displays of anti-nuclear activism and opposition to Britain’s efforts to keep pace with other military powers. For Ms. Arrowsmith, the march also put her on the map for British authorities. She claimed she was under surveillance for decades by agents from MI-5, the country’s domestic intelligence agency. “It leaves one feeling rather suspicious,” she said in a 2002 interview.
Her published works often reflected the tensions and frustrations of fighting the system. The 1965 novel “Jericho” is based on the Aldermaston march and “Somewhere Like This” (1970) is set in a women’s prison. (She often joked that being an inmate was a “cinch” after the rules at the Cheltenham school, where she was almost expelled for sneaking away in 1945 to celebrate V-E Day.) A nonfiction book, “To Asia in Peace” (1972), recounts her time in Vietnam and Cambodia.
A memoir in 1993, “I Should Have Been a Hornby Train,” displayed some of her self-deprecating wit. The title refers to the dismay of her two brothers after she was born — hoping their parents would bring home model trains rather than a baby sister.
Margaret Pat Arrowsmith was born March 2, 1930, in Leamington Spa in Warwickshire, southeast of Birmingham. Her father was an Anglican clergyman; her mother was a homemaker and daughter of Plymouth Brethren missionaries in China who were killed during the xenophobic Boxer Rebellion at the turn of the century.
Ms. Arrowsmith’s mother was hidden under a bed by her Chinese caretaker and taken out of the country.
Ms. Arrowsmith received a history degree from Cambridge in 1951. She earned a certificate in social science in 1955 from the University of Liverpool and went on to work as a social caseworker in the city for two years.
She was a co-founder of the Campaign for Nuclear Disarmament and had various roles with the rights group Amnesty International from 1970 to 1994. Amnesty twice designated Ms. Arrowsmith a prisoner of conscience during her times in jail. She made three unsuccessful runs for Parliament, launching the last bid in 1979 against then prime minister, James Callaghan, for a seat in Cardiff, Wales.
As a young woman, she openly described herself as lesbian. Her father was dismayed and put a provision in his will that she could not receive inheritance unless she was married. After her father’s death in 1976, Ms. Arrowsmith wed poet Donald Gardner and had the marriage annulled the same day. She donated some of her inheritance to various groups including gay and lesbian organizations.
She had no immediate survivors.
Ms. Arrowsmith didn’t limit her fights to the big issues of war and weapons. She once took up the cause of pigeons. “Animal murder,” she declared in 2001 after London’s mayor, Ken Livingstone, proposed limits on the public tradition of feeding pigeons in Trafalgar Square. Ms. Arrowsmith tossed her bird feed and told reporters that she was prepared to be arrested once again. This time, she wasn’t.
