Ms Chacón was found dead at her Madrid home, aged 46, after a neighbour had raised the alarm. A congenital heart problem was the likely cause of death.
Her son Miquel is eight years old.
Spanish politicians were among the many Spaniards who expressed shock and sympathy over Ms Chacón’s sudden death.
Mr Zapatero said he was “heartbroken at the death of my comrade and friend”.
He said “we feel emptiness, because of the enormity of this loss, for Spanish democracy and for all socialists”.
At the age of 37, and seven months pregnant, she visited Afghanistan, five days after being appointed defence minister.
“A pregnant woman is not sick. Sure, it’s harder when you’re pregnant and on your feet all day. I wanted to express society’s gratitude to those who put themselves in danger to bring peace to regions of conflict.”
After the Afghanistan trip she also reviewed troops in Madrid while pregnant in May 2008.
She joined the Socialist Youth at the age of 18, and began her political career as a local councillor in 1999-2007. Later she served as an MP representing a Barcelona constituency. She was born in Llobregat, Barcelona, in 1971.
She was the daughter of a fire service official in Barcelona and his lawyer wife. As a girl she had been advised to lead a calm life because of her heart condition. But she ignored the advice repeatedly – including the suggestion that it would be risky to have children, El Pais reports.
“Spain is losing a distinguished figure from its modern political scene. And Spanish women are losing someone who always took a stand to emphasise the importance of women’s roles in modern society, and among the college’s faculty and students.”