Paramedics were called to the singer's home around midday local time after he suffered a suspected cardiac arrest.
He was rushed by ambulance to a medical centre, with reports from several US media outlets saying the 50-year-old star has died.
Michael Jackson had been due to begin a series of comeback concerts in the UK on 13 July.
He has a history of health problems and has not completed a concert tour in 12 years.
US media reports say paramedics were called to the singer's house in Bel Air at midday.
They performed CPR on Jackson and rushed him to the UCLA medical centre.
Crowds have begun to gather outside the facility, whose emergency centre has been roped off by police.
Jackson began his career as a child in family group The Jackson 5.
He then went on to achieve global fame as a solo artist with smash hits such as "Thriller" and "Bad".
But the singer has been dogged by controversy and money trouble in recent years, becoming a virtual recluse.