Boxing greats paid their respects to former heavyweight champion Joe Frazier during a private funeral at the Enon Tabernacle Baptist Church, in Philadelphia.

'Smokin' Joe' fought with fearless aggression that belied his modest size and packed a powerful left hook that helped him rule the world heavyweight division from 1970 to 1973.

Frazier, the first boxer to beat Muhammad Ali, died in Philadelphia a month after being diagnosed with liver cancer.

Ali and the Reverend Jesse Jackson were two friends in attendance, paying their last respects.

The Rev. Jesse Jackson asked mourners to "show love" for the former heavyweight champion.

"Tell them Rocky was not a champion. Jose Frazier was," he said, referring to the hometown character from the boxing movie, Rocky, and whose statue stands at the base of the Philadelphia Museum of Art.

"Tell them Rocky is fictitious, Joe was reality. Rocky's fists are frozen in stone. Joe's fists are smokin'. Rocky never faced Ali or Holmes or Foreman. Rocky never tasted his blood. Champions are made in the ring not in the movies. There deserves to be a statue of Joe Frazier in downtown Philadelphia."

Frazier's career took flight when he won the Olympic heavyweight boxing gold medal for the United States in 1964 in Tokyo. Thereafter he went on to hold the world heavyweight boxing crown from 1970 to 1973.

Frazier won the world heavyweight title in 1970, knocking out champion Jimmy Ellis, after Muhammad Ali had been stripped of the championship in 1967 for refusing to fight in the Vietnam War due to his Muslim beliefs.

Ali was later reinstated into boxing and his very first fight was that famous one against Frazier.

Frazier will always be remembered for his rivalry with Ali -- a rivalry is etched in history, the sort of stuff legends are made of.

Joe Frazier had to wait until death to win rightful appreciation after a boxing career spent in the shadow of the magnificent Muhammad Ali, boxing historian Bert Sugar said.

"He never got his due," Sugar said, when asked about the former world heavyweight champion, who fought three epic bouts with Ali in the 1970s.

"People say Ali-Frazier, and the winner of that fight was Frazier," Sugar said about their first clash in March of 1971, a battle of undefeated heavyweights at Madison Square Garden that left both boxers hospitalised.

"This is the day for Joe Frazier, who has always been the second slot to Muhammad Ali, even when he won. Today is the day when he stands up as his own and he's Joe Frazier, period, paragraph, and he can stand up to anybody on his own."