Nestor Kirchner, former president of Argentina and husband of the current leader, died Wednesday after suffering a heart attack, the presidency said.
Kirchner died as he and President Cristina Fernandez waited in their home in the Patagonian city of Calafate to be counted in the nation's census.
Fernandez was by his side when he died, state television reported.
"It was a sudden death," his personal doctor, Luis Buonomo, told reporters at the Forementi de Calafate hospital. He said an official medical report would be released later in the day.
Kirchner, 60, had undergone an angioplasty after a heart attack in September.
He was still a likely candidate in next year's presidential elections and was secretary general of the South American alliance known as Unasur. He also served as a congressman and leader of the Peronist party.
The news of his death shocked Argentines.
"A great patriot has died," said Juan Carlos Dante Gullo, a ruling party congressman, to state TV. "This will leave a huge hole in Argentine politics. We will have to follow his example. Argentina has lost one of its greatest men."
The leader of the human rights group Grandmothers of the Plaza de Mayo, Estela de Carlotto, said Kirchner "gave his life for his country."
"Our country needed this man so much. He was indispensable," she told Radio Continental.
Kirchner was president from 2003 to 2007, bringing Argentina out of severe economic crisis and encouraging changes in Argentina's justice system that set in motion dozens of human rights trials involving hundreds of dictatorship-era figures who had previously benefited from an amnesty.
He recently was appointed secretary general of the Union of South American Republics, or Unasur, and was preparing for an intense 2011 election campaign in which either he or his wife would run again.
Born in the Patagonian province of Santa Cruz, Kirchner and his wife were active in the Young Peronists party as students in La Plata, where he graduated in 1976.
With the military firmly ruling the country, the young couple married and worked as private lawyers in the provincial capital. After democracy returned in 1983, Kirchner entered public service, first as the provincial pensions chief and then as mayor of Rio Gallegos.
In 1991, he became Santa Cruz's governor and Fernandez was elected to the provincial legislature, pushing through indefinite re-election and filling the provincial courts with sympathetic judges. In 1995, he was re-elected as governor by an overwhelming margin, laying the groundwork for a jump to politics at the national level.