Ali, a true original, was the most celebrated heavyweight boxer of all time.
PHOTOS: Muhammad Ali through the years
The Rev. Jesse Jackson, visiting the Bay Area, called Ali a longtime close friend.
"My heart is so heavy," Jackson said. "A champion in the ring, a hero outside the ring, a social transformer, an anti-war prophet."
Al was last seen publicly in April looking frail and drawn. For the last 30 years, he's been afflicted by Parkinson's disease, the possible toll of so many blows to head in the ring. His fight is bringing greater awareness to the disease.
Mayor of Louisville, KY, Muhammad Ali's hometown, says flags at gov't buildings will be lowered to half-staff. pic.twitter.com/VNUZRZ3BhG— ABC News (@ABC) June 4, 2016
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"A man who sacrificed his own dignity. Muhammad Ali has made us better," Jackson said.
Novato Boxing Club owner Susan Howard calls Ali her hero. His pictures and fight posters cover the walls of her gym.
"It's sad because he's somebody I've followed my whole life," she said.
Howard met Ali at a book signing in Berkeley in the 90s, but she never forgot how the boxing legend commanded the room.
"I saw him and I went, wow, took my breath away," Howard said.
Ali's life led Howard to her own career in pro-boxing and kickboxing. She learned a lot from the man who invented self-promotion.
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"He was trying to draw people in, and did a great job," she said.
Reaction to Ali's death was quick across the Bay Area.
"I think His boisterous character played off well and I see him as an influence to the world," said San Francisco resident Stephen Sacks.
"It was sad to see his quality of life diminished in the last couple of decades," said Laura Varga of Burlingame.
"I can recall my childhood. He became a hero, now in an era where there's not many heroes anymore," said Robert Houston of Sacramento.
A boxer and legend, forever.
Muhammad Ali. The Greatest. Rest in Peace. pic.twitter.com/Rji6KryXeN— San Francisco Giants (@SFGiants) June 4, 2016
Today we lost a boxer, activists, poet & the conscience of a nation! Meeting him was one of the highlights of my life! #RIPAli— Adonal Foyle (@afoyle3131) June 4, 2016
The Greatest is gone ... but a timeless image lives forever.— ESPN (@espn) June 4, 2016
(John Rooney/AP) pic.twitter.com/BPFcAyNUTM
Muhammad Ali, "The Greatest," has died at the age of 74. pic.twitter.com/xoR8EqRl5d— ESPN (@espn) June 4, 2016