"An absence of blood flow in the brain was identified," doctors at the Sirio-Libanes hospital said, declaring Marisa Leticia Lula da Silva brain-dead.
The family of da Silva, 66, had authorized the donation of her organs and "procedures were begun for the donation," the hospital said in a statement.
Lula's family issued a statement thanking supporters for "all the expressions of sympathy and solidarity."
Da Silva had been treated in intensive care since January 24 with a brain hemorrhage due to a ruptured brain aneurism.
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Although Lula left power with sky-high ratings and taking credit for Brazil's economic boom, his legacy has been badly tarnished by a subsequent recession and a string of corruption charges last year in which his wife was also indicted.
Both Lula and his wife denied any wrongdoing.
Lula's ally and successor in the presidency, Dilma Rousseff, described da Silva as "a fighter" and said "she had an important political role."
"They experienced great injustices. I imagine that Lula's pain now is intolerable," Rousseff said. "We are together president Lula, now and forever."
Joao de Oliveira, a former head of the CUT trade union federation, praised da Silva's history as one of the original members of the Workers' Party Lula founded.
"She made the Workers' Party's first red-and-white flag," he said. "We have high regard for her because she gave her whole life to build the partisan movement, the union movement.
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