Immediately, Marín, a leading member of the Chilean Communist party and a parliamentarian, broadcast a desperate message of defiance on Radio Magallanes. Her name appeared on the junta's most-wanted list, and she went underground, separated from her husband, the Santiago Communist party secretary Jorge Muñoz, and their two sons.
Ordered by the party to leave the country, Marín flew to Moscow in November, leaving her sons with their grandparents. Her husband remained on the run until he was arrested, with other communist leaders, in 1976. Marín was in Costa Rica when she heard that he had "disappeared" in secret police custody. His body was never found.
In 1978, Marín slipped back into Chile, and spearheaded the communist operation to infiltrate exiles to develop the resistance. Even her sons were unaware that she was back. Once, while lodging in a house on the street where they lived, she recognised one of her children, but was too afraid for his safety to give him the embrace she yearned for.
In 1980, the Communist party opted for a strategy of popular rebellion, and Marín was instrumental in the creation of its armed wing, the Manuel Rodríguez Patriotic Front (FPMR), led by returned exiles with military training. Its most spectacular action was an abortive assassination attempt on Pinochet in 1986.
Marín was born in Curepto. Her mother, Adriana, was a schoolteacher, and her father, Heraclio, a small farmer and travelling salesman. Adriana left after discovering that her husband had fathered a number of the children she taught. Marín lived with her mother in Sarmiento and Talagante, south-west of the capital, Santiago. There, she would gather firewood in the hills.
A move to Santiago brought her to the Escuela Normal, where she joined the student union. When, in 1958, the Communist party was legalised in Chile, Marín joined the Communist Youth and supported the socialist Allende's unsuccessful presidential campaign.
The following year, she met Muñoz, then a mining engineer, and they married within a year. They worked in Allende's third campaign in 1964. A year later, Marín became Chile's youngest parliamentarian, and, between 1966 and 1977, she was general secretary of Communist Youth.
Allende's Popular Unity coalition won power in 1970. Marín took part in much of the left's extra-parliamentary work - peasant movements, voluntary urban work crews and cultural groups. The government, meanwhile, faced formidably funded rightwing subversion. The army chief-of-staff was assassinated, and, in 1973, his successor was ousted in favour of Pinochet.
By the mid-1980s, the Communist party's Popular Unity allies had opted for a coalition with the Christian Democrats, a move that Marín saw as a betrayal, although the regime undertook a limited transition to democracy.
Marín resurfaced from clandestinity in 1990, and, four years later, was elected party general secretary. In 1996, she was charged with "criminal calumny" for denouncing Pinochet as a tyrannical coward and psychopath, and jailed for four days. In 1998, while he still headed the armed forces, she became the first person to file criminal charges against him. When he took up his self-awarded senate seat, Marín was one of many protesters beaten up by paramilitary police outside parliament.
In 1999, she stood as the first Communist party presidential candidate in Chile since 1932, though with only 225,000 votes she came a distant third. When, in 2001, paramilitary police evicted the party from its offices, Marín resisted the takeover with a team of lawyers, cheered on building workers who hurled bricks at the police and was injured while defending the premises.
She gave up the general secretaryship in 2002, and was elected party president. Her autobiography, La vida es hoy (Life is Today), was published in 2003.
As the gravity of her illness became known, Marín won affection in even the conservative Chilean media, and was dubbed "everyone's favourite red". The communists enlisted leading artists in a fundraising campaign to pay for her treatment and re-equip the country's health service. The appeal mobilised an extraordinary array of social groups, from Christian leftists to the indigenous Mapuche community - Marín had a profound respect for popular religious sentiment.
She received post-operative care in Cuba, where she dictated more memoirs and wrote for the party press, but deliberately returned to Chile to die. Death, she once said, held no fear for her; she had spent half of her life in its shadow. But "La Gladys", as even her sons called her, lived to see Pinochet arraigned before a Chilean court.
She is survived by her partner, the journalist Julio Ugas, and her sons.
- Gladys Marín Millie, revolutionary, born 16 July 1941; died 6 March 2005