Jackson's death, just weeks before he was due to start a 50-date tour of the UK, has sent shockwaves around the world.
Rev Jesse Jackson was among those paying tribute to the music icon.
"He tried so hard to get back and he was ready," he told US channel CBS.
Rev Jackson had known the troubled star since his childhood years as a singer with The Jackson Five and previously spent time counselling him at his Neverland ranch in southern California.
He said: "We've lost our joy, he's lost his pain. And he was so frightened by it and so fragile."
US civil rights campaigner and close friend of Michael Jackson, the Rev Al Sharpton, said he was a "trailblazer" who had helped many people through his charities.
"I feel like he was not treated fairly. I hope history will be more kind to him than some of the contemporary media," he said.
Celebrities have also been paying tribute to the star.
Madonna said: "I can't stop crying over the sad news. I've always admired Michael Jackson – the world has lost one of its greats but his music will live on forever.
"My heart goes out to his three children and other members of his family. God bless."
"As a friend of Michael's for the past 35 years, I call on people from around the world to pray for him and his family."
Rev Al Sharpton, civil rights campaigner
Britney Spears said: "I was so excited to see his show in London. We were going to be on tour in Europe at the same time and I was going to fly in to see him. He has been an inspiration throughout my entire life and I'm devastated he's gone!"
Jackson rose from childhood stardom in The Jackson Five to become one of the most influential singers, songwriters and performers in pop history. In addition to his music, he dazzled generations of fans with his moonwalking and thrilling onstage performances.
In his 40-year career, the "King of Pop" sold an estimated 750 million records worldwide, revolutionising pop music and winning 13 Grammy Awards. In 1982, he enjoyed phenomenal success with his hit album "Thriller" before going on to release "Bad" in 1987.
The last two decades saw Jackson's gradual demise and increasingly erratic behaviour. In 2002 he shocked the world when he dangled his baby son Prince Michael II over a hotel balcony. Then in 2005 he was arrested and tried for allegedly sexually molesting 13-year-old Gavin Arvizo. He was cleared of the charges.
Distraught fans have gathered outside the UCLA Medical Center and the Jackson family home in Encino, California.
A post mortem examination is due to be carried out today to establish the cause of death.