The services for Ali are open to the public and will be held in the three-time heavyweight champion's hometown of Louisville, Ky. Ali died of septic shock from unspecified causes Friday night at the age of 74.
Smith portrayed the legendary boxer in the 2001 movie "Ali", earning a nomination for Best Actor by the Academy Awards.
Lewis, who was also a fellow Olympic champion, shared his feelings on his official website.
"Words cannot begin to describe the loss we all face in the passing of one of the greatest citizens this world has even known," Lewis wrote. "Boxing has lost its greatest ambassador and the world has lost one of its greatest voices."
Family members and friends will also serve as pallbearers, including Jerry Ellis, the brother of Ali's former sparring partner and heavyweight champion Jimmy Ellis.
Ali, who retired from boxing in 1981, battled Parkinson's disease since being diagnosed in 1984 at age 42.