The founder of Portugal's Socialist party, Soares spent decades in politics and spearheaded the country's entry into the European Union. He was president from 1986 to 1996 after serving as foreign minister and prime minister, and later became a European lawmaker.
Portugal declared three days of national mourning from Monday for the deceased leader.
"We have lost today someone who has so many times been the face and the voice of our freedom, for which he fought all his life," said the country's current prime minister and fellow socialist Antonio Costa.
Soares had been admitted to hospital on December 13, and although his condition initially showed signs of some improvement, he later fell into a deep coma from which he never recovered.
The hospital did not reveal the precise cause of Soares's death, but relatives say he never fully overcame a spate of illness in 2013. His health further deteriorated after his wife's death in July 2015.
The new United Nations secretary-general, ex-Portuguese premier Antonio Guterres, hailed Soares as "one of the rare political leaders who had real stature in both Europe and the world."
Portuguese President Marcelo Rebelo de Sousa delivers a speech after it was announced that former Portuguese President, Mario Soares has died. Source: LUSA
'Fighter for freedom'
Born in Lisbon on December 7, 1924, Mario Alberto Nobre Lopes Soares was raised in a family opposed to the dictatorship of Antonio Oliveira Salazar.
His father Joao Soares, a defrocked priest, struggled against the regime for decades, suffering long periods of imprisonment and exile.
Reassuringly portly, Mario Soares was both a charmer and a humanist known for being spontaneous and warm.
A self-defined agnostic, he Soares said he believed in "humanity and its improvement", and described himself as being driven by "a great desire to live and by immense curiosity."
"I am a poor man who has been fortunate to have taken stands and to have been right," he told the "i" daily in February 2015, rejecting the idea he was an "immortal" figure.
Several politicians, including Costa and President Marcelo Rebelo de Sousa had visited Soares in hospital in recent weeks and news his death brought emotional tributes from all political sides.
"He was a fighter for freedom" and Portugal must henceforth fight for "the immortality of his heritage," said conservative leader Rebelo de Sousa.
"It is a sad day for all Portuguese," added the head of the centre-right opposition Pedro Passos Coelho.
And that sorrow could be seen on the streets of Lisbon Saturday.
"His death saddens me. Even in his old age, he was someone who said what he thought," said Paula Cabecadas, a 60-year-old bookseller.
"For me he was like a dinosaur: this huge figure from the past who will be hard to match in the future," said 22-year-old student Miguel Pinto.
Critic of austerity
On the international stage, Soares was also seen as a political giant, "a great European" and the "decisive figure for Portuguese democracy," said Spanish Prime Minister Mariano Rajoy.
Soares was "the symbol and the artisan of resistance to the dictatorship and the transition of his country to democracy," said European Commission chief Jean-Claude Juncker.
French President Francois Hollande said Portugal's democracy had lost "one of its heroes, Europe, one of its great leaders and France, a faithful friend".
While Soares retired from public life for several months after a crushing presidential election defeat in 2006, he appeared regularly in the media to comment on current affairs.
He emerged as a fierce critic of the steep spending cuts Portugal was forced to implement under a 78-billion-euro bailout deal reached in 2011 with the EU and IMF to avert bankruptcy.
Soares accused big European nations at the time of being guided by "savage capitalism".
He made a final public appearance in July, when he attended a ceremony held in his honour by the ruling Socialist-led government.
Visibly frail, he did not make a speech.
