His daughter-in-law Catherine Zeta-Jones led the tributes, writing: "To my darling Kirk, I shall love you for the rest of my life. I miss you already."
Tributes also came from director Steven Spielberg, Star Wars actor Mark Hamill and Breaking Bad star Bryan Cranston.
Douglas, who played the title role in the 1960 classic Spartacus, enjoyed a career that spanned seven decades.
Spielberg said Douglas left behind a "breathtaking body of work".
He told The Hollywood Reporter: "Kirk retained his movie star charisma right to the end of his wonderful life and I'm honoured to have been a small part of his last 45 years."
Jamie Lee Curtis, whose father Tony was also in Spartacus, declared: "He LOVED you as the world loved you. Your Passion. Talent. Politics. Family. Art. Strength."
Hamill described Douglas as "one of the biggest stars of all time", as well as "a brilliant actor with an unforgettable, blazing charisma".
He also referenced Douglas's role in ending the 1950s Hollywood blacklist by defying the ban on working with film-makers with alleged communist sympathies.
Rob Reiner, who directed films including This Is Spinal Tap and When Harry Met Sally, described him as an "icon in the pantheon of Hollywood".
The Hollywood veteran's death was announced on Wednesday by his son, fellow actor Michael Douglas.
"To the world he was a legend, an actor from the golden age of movies who lived well into his golden years," he wrote on Instagram.
Photo supplied Caption: Micheal Douglas (left) with dad Kirk