Speaking at a reception for veterans of the 1966 operation, Renzi said the 10,000 mainly young volunteers who came from all over the world were still a source of inspiration.
"Your optimism and your energy leads us to look to the future with enthusiasm," said Renzi, a former mayor of the city.
Friday's ceremonies marked exactly 50 years since the Arno river burst its banks, flooding the city and surrounding areas in a disaster that left at least 34 people dead.
When the waters receded, vast fields of mud were left and every town house, church and museum had been completely saturated, sometimes up to the first floor.
But thanks to the efforts of the volunteers, thousands of paintings and millions of books were cleaned up and saved for future generations.
Among the artworks submerged by the flood was a giant tableau, "The Last Supper", by Giorgio Vasari (1511-1574).
Its restoration to its full glory has only recently been completed and it was due to be returned to its original home in the Basilica of Santa Croce on Friday, as a symbolic celebration of the remarkable display of cultural