The boxing legend was speaking at a Canadian Football League game in Toronto.

Ali was there to raise funds for research into Parkinson's disease, the affliction which has blighted his life since he retired from boxing.

"I'm now no longer the greatest," he told the crowd.

"He's the greatest, he's the champ," he said, pointing to Lewis, who crushed Mike Tyson in his last fight in June.

Greeted with chants of "Ali! Ali!" from the 25,000-strong crowd, Ali, 60, showed he had lost little of his sharp wit nor his showman's instincts though his body has been slowed by Parkinson's disease.

He mock sparred with Lewis before saying: "Don't worry, I wasn't going to hurt him."

Two other heavyweight greats, Larry Holmes and Evander Holyfield, were in attendance to pay tribute to Ali, the first man to win the heavyweight title on three occasions.

Ali's appearance, broadcast live across Canada, was also aimed at boosting dismal attendance records for Toronto Argonaut football games which have averaged 20,000 per game.

He also told reporters his illness was God's way of challenging him.

"God has ways of testing people and he can pull you down. And when you're down, you keep praying. It's a big test," he said.

"(You have to) take your medicine all the time and don't give up.

"I give up sometimes, thinking I don't need this stuff (medicine) and have a hard time walking, a hard time talking."