Kyrkos, who was born in Irakleio, Crete, in 1924, was one of the founders of the 1950s-era leftist EDA party, president KKE (internal branch) party as well as a co-founder of the early 1990s Coalition of the Left & Progress.
He had served several times as a Parliament MP and MEP. Kyrkos was jailed by the military junta in 1967 for five years.
Condolences and statements in memory of Kyrkos poured in on Sunday by political party representatives.
His funeral is scheduled for Wednesday at the First Athens Cemetery.
President Karolos Papoulias, in a message, noted that "Leonidas Kyrkos was a historic political personality of the country, who devoted his life to the consolidation of democratic ideals and the stability of parliamentarism ... We shall remember him for his spirit of renewal and for the accuracy of his speech."
Prime Minister George Papandreou, in commenting on Kyrkos' passing, stressed that "he was one of the most distinguished personalities of the left; a fighter for democracy and freedom and a pioneer voice for reform-minded ideas; he was a visionary of a united Europe, one of progress and social justice."
Parliament President Filippos Petsalnikos spoke of "a distinguished personality of politics, who marked with his action the course of the country, serving freedom, democracy and human dignity."
Main opposition New Democracy party leader Antonis Samaras referred to Kyrkos' "rich political heritage for all, friends and rivals ... a leader in the movement for the renewal of the Left, who was fortunate to be honored in life by many rivals and to be loved by many friends of his, from all the sides of the political spectrum."
Government Vice President and Finance Minister Evangelos Venizelos stressed that "with his personal course, Leonidas Kyrkos has left his mark on history. A personality that will be always present, in a political environment that lacks, unfortunately, generosity and consensual political proposals."