Jose Eduardo Dos Santos, who ruled Angola for with an iron fist for 38 years, died Friday at a hospital in Barcelona after suffering cardiac arrest, the government said.
With "great pain and consternation" the Luanda government posted on Facebook confirmation of the Dos Santos' death at 11:10 am (1010 GMT). He was 79.
"(The government) presents its deepest feelings of sorrow to the bereaved family," the statement read, describing the former leader as a "statesman of great historical stature" who led the country through very difficult times.
Angolan President Joao Lourenco, who is seeking re-election in August, declared five days of national mourning, starting on Saturday, and set-up a government commission to organise funerals.
A presidential decree ordered flags to be flown at half-staff and the cancellation of "all shows and political demonstrations".
But one of Dos Santos' daughters demanded an autopsy be carried out on the body on suspicion of foul play.
While the former president was lying in hospital earlier this week, Tchize Dos Santos filed suit with the Catalan regional police, alleging her father's condition was the result of attempted murder.
According to the complaint, she believed her father's wife, Ana Paula, and his personal physician were responsible for the deterioration in his health, one of her lawyers said.
Dos Santos stepped down in September 2017 after 38 years at the helm of the Portuguese-speaking, oil-rich state of Angola.
He was hospitalised in Spain and placed in intensive care after suffering a cardiac arrest on June 23.
Born in the slums of Luanda, he was one of Africa's longest-serving leaders, using his nation's oil wealth to turn one of his children into a billionaire while leaving his people among the poorest on the planet.
During his reign, he avoided the personality cult so often favoured by dictators, but instead used secretive and authoritarian tactics he learned during the Soviet era.
And for as much as he controlled every aspect of Angolan life, he mismanaged his own transition away from power so badly that he ended up in temporary self-imposed exile, with a son in prison and a daughter facing international legal challenges.
When he stepped down, dos Santos handed over to former defence minister Lourenco, handpicked to replace him.
But Lourenco quickly turned on his erstwhile patron, starting an anti-corruption drive to recoup the billions he suspected had been embezzled under dos Santos.
Dos Santos' eldest daughter Isabel was once named by Forbes as Africa's richest woman, worth $3 billion (2.55 billion euros). She now faces a slate of investigations into her multinational business dealings.