The founder of Portugal's Socialist Party, who served as president from 1986-96, died in hospital on Saturday.
Monday was the start of three days of national mourning.
The funeral cortege was to leave his family home in Lisbon, where mourners laid red roses, the Socialist Party symbol, on a journey to the capital's 16th century Hieronymite monastery.
It was there that Soares, then prime minister and soon-to-be president, signed Portugal's treaty of accession to what was then the European Economic Community, a forerunner of the European Union.
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He will be buried on Tuesday afternoon at the Prazeres cemetery in the west of the capital, alongside his wife, actress and philanthropist Maria Barroso, who died in July 2015.
Portugal's Prime Minister and fellow Socialist Antonio Costa said Saturday that the country had lost "someone who has so many times been the face and the voice of our freedom, for which he fought all his life".
"It's because of him that we escaped communism, Portuguese people didn't want it," said Maria Albuquerque, a 76-year-old shopkeeper who came to sign a condolence book at the Socialist Party headquarters.
The hospital did not reveal the precise cause of Soares's death, but relatives say he never fully overcame a spate of illnesses in 2013. His health further deteriorated after his wife's death.
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