Abraham Osheroff was an American veteran of the Spanish Civil War and a dedicated social activist. In Spain he fought with the Abraham Lincoln Battalion against General Francisco Franco’s nationalist forces.

That was not his first or his last political act, however, and he achieved lifelong notoriety as a left-wing militant with an outspoken manner and tireless energy. He produced two documentaries on Spain and delivered countless speeches, the last as a nonagenarian only a week before his death. “The bastards will never cease their evil,” he said at this last event, describing his motivation, “and the decent human beings will never stop their struggle.”

Osheroff believed in what he called “radical humanism” and practised it at every opportunity, campaigning vigorously against repression and militarism. During the “Freedom Summer” of 1964 he helped to build a community centre in a black Mississippi district, where his car was firebombed by white supremacists. In the 1980s he co-ordinated a construction project for a Sandinista peasant co-operative in Nicaragua. Osheroff was most recently arrested in 2006 for taking part in a sit-in protest against the Iraq war in a senator’s office. He even fought property developers over California’s canals.

Abraham Osheroff was born in Brooklyn, New York, in 1915, the son of Russian-Jewish immigrant parents. He grew up in Brownsville, a working-class neighbourhood with strong socialist roots, and English was his third language.

During the Great Depression he helped evicted tenants to move their belongings back into their rented apartments in defiance of landlords and the police. When he was arrested by the latter, he was beaten and called a “dirty Communist Jew bastard”. “All of which was true,” Osheroff conceded, “except for the bastard part.”

He studied philosophy at City College, New York, but followed his calling and became a mining union organiser in Pennsylvania and Ohio.

When the Spanish Civil War broke out in July 1936 he instinctively sided with the Republic against the military rebels. Like many of his International Brigade comrades, the event that triggered the decision to head for Spain was the images of the German and Italian air forces indiscriminately bombing civilians at Guernica, in the Basque Country.

Osheroff sailed for Europe in May 1937. From Marseilles he was due to travel to Barcelona by ship because the land border was closed under the international non-intervention agreement. However, an Italian submarine torpedoed the ship about two miles off the coast of Catalonia.

Osheroff was a strong and confident swimmer. As the ship began to sink, he dived off the side and headed for shore, although he quickly regretted abandoning other survivors, some of whom could not swim. He turned back and helped them on to floating wreckage, from where they were rescued by Catalonian fishermen. Osheroff himself did not wait for assistance and swam ashore alone; 70 of his fellow passengers were killed.

In Spain Osheroff joined the Lincoln Battalion and served as an infantryman. He fought in four significant battles. On leave in Barcelona he and a friend met Ernest Hemingway, and accepted an invitation to join the writer and reporter for a drink in his hotel room. Hemingway’s cupboard was full of tobacco, whisky and chocolate. These luxury items were not easily available at the front, and Osheroff decided to help himself and share them with the Lincolns. The pair stuffed their pockets and beat a fighting retreat. Hemingway had a “good right cross”, according to Osheroff.

Osheroff was wounded in the Battle of Quinto in September 1937, near Zaragoza. As he initiated a charge at the enemy, in an attempt to rally the troops, he was shot in the knee. “I was brought down by machinegun fire. My left leg was smashed pretty bad,” he said. He returned to the US in August 1938.

Back in Brooklyn a stiff-legged Osheroff consulted a doctor who, by chance, shared his left-wing sympathies. Hearing that Osheroff’s limp was the result of a fascist bullet, the doctor agreed to operate on the knee without charge. Osheroff also worked as a carpenter in Brooklyn and in 1940 unsuccessfully ran for the New York legislature as a communist.

He served in the US Army during the Second World War and took part in operations in France after the D-Day Normandy landings. Conditions in the lavishly equipped American forces were a “lark” compared with the civil war, he recalled. “I never missed a meal, I had good shoes, I had tremendous medical attention . . . and we had none of that in Spain,” he said.

During the Cold War he became suspect in the US because of his politics. In 1949 he went semi-underground after a tip-off that he was on an FBI wanted list of communists. He moved around the country and between jobs until he settled in Los Angeles.

Throughout this period he became disillusioned with the Communist Party and, after the Soviet Union’s repression of the Hungarian revolution in 1956, he renounced his membership. He came to be regarded as a critical independent thinker and not an apologist of the Left.

In 1971 Osheroff returned to Spain where, despite his lack of experience, he duped the authorities and filmed an award-winning documentary. Dreams and Nightmares (1974) portrayed Osheroff’s war — “one of the biggest crises of my life” — and a disappointing return to a country still under Franco’s regime. The documentary’s release provided an opportunity for him to travel the US and lecture extensively on the civil war.

In 2000 he produced a second documentary, Art in the Struggle for Freedom, on Spanish Civil War poster art and poetry.

As Osheroff’s age began to catch up with him, he devised a way to continue his activism: the Peace Mobile. He fitted a van with posters and a loudspeaker that allowed him to remain vocal while sitting down. “My ship is slowly sinking,” he used to say, “but the cannons keep firing.”

He lived to see the first memorial to the Lincoln Battalion unveiled in the US. From a wheelchair he delivered his last rallying speech at the event on March 30, 2008, in San Francisco.

Osheroff was married three times. He is survived by his long-term partner, Gunnel Clark, his daughter and two sons.

Abraham Osheroff, International Brigade veteran and political activist, was born on October 15, 1915. He died on April 6, 2008, aged 92