A CNN affiliate says he was hospitalized just before 12:30 p.m. Pacific Standard Time after suffering cardiac arrest.
Affiliate KTLA reports that Jackson was not breathing when emergency responders arrived and paramedics performed CPR.
The Los Angeles Times reported that he arrived at UCLA Medical Center in a deep coma. Crowds of mourners congregated Thursday night in front of the hospital.
Jackson gained national attention as a youth in the 1960s, where he performed at the Apollo Theater as a member of The Jackson 5.
Fans gathered Thursday evening in front of the theater to remember the performer and play his music.
He rose to international fame in the 1980s as the "King Of Pop," known for his innovative choreography which included the "Moonwalk" dance step. Nearly three decades after its release, his 1982 album "Thriller" remains one of the best-selling albums of all time. He later became a tabloid news standard with allegations of frequent plastic surgeries and charges of child molestation.
Fans who gathered in Times Square, including many who grew up listening to Jackson's music and watching his videos, said that they were shocked and saddened by the singer's sudden demise.
"I was thinking it's the end of an era," said one fan.
"It's sad. He's an '80s icon," said another.
"I grew up listening to his songs. There's no place that you can go where you listen to Michael Jackson that you don't start dancing, and that's sad."
The Reverend Al Sharpton of Harlem, who said he was Jackson's friend for 35 years, said in a statement, "I have known Michael since we were both teens, worked with him, marched for him, hosted him at our House of Justice headquarters in New York, and we joined together to eulogize our mutual idol, James Brown. I have known him at his high moments and his low moments and I know he would want us to pray for his family."
Jackson was 50 years old.