Cleaver died at 6:20 a.m. PDT at Pomona Valley Hospital Medical Center, spokeswoman Leslie Porras said. She declined to provide any cause of death or any details about his hospitalization.
"Per the family's request, that information is not being released," she said.
Family members who had been with him "are no longer all here at the hospital," she said.
At times a convict, political candidate and author, Cleaver was one of the original Black Panthers, formed in 1966 in Oakland by Huey P. Newton and Bobby Seale.
The Black Panthers, often dressed in black berets and black leather jackets, also espoused militant revolutionary and survivalist tactics. As it matured, the party became a national organization that ran food programs and schools for underprivileged youth.
Cleaver, the fledgling party's information minister, was involved in a violent April 6, 1968, shootout with police in Oakland. Panther treasurer Bobby Hutton, 17, was killed in the gun battle, and Cleaver and two police officers were wounded.
Cleaver was arrested after the shootout, but jumped $50,000 bail and fled the United States.
Prior to his return to the United States in 1975, he told reporters he believed he would be treated fairly in court.
"A new situation now exists in the United States. The war in Vietnam is over. The status and condition of black people has undergone a fundamental change for the better. The American people have been shocked into objectivity and vigilance by the exposure of the massive, systematic and conspiratorial subversion of their democratic rights," he said.
He also denounced the Black Panthers upon his return.
After a protracted legal battle, attempted-murder charges in the shootout were dropped, and Cleaver was placed on probation and ordered to do community service for assault.
In 1958, before his Black Panther days, Cleaver was convicted of assault with intent to kill. He was paroled after nine years in prison. He also had a 1954 conviction for narcotics possession.
While in prison, Cleaver wrote Soul on Ice, a series of powerful essays decrying prejudice and racism. Published in 1968, it became a focus of the Black Power movement.
Cleaver ran unsuccessfully for president in 1968 on the Peace and Freedom Party ticket. Following his campaign, he was ousted from the Panthers because of a bitter and public dispute with Newton.
After his self-imposed exile in Algeria and France, Cleaver became a born-again Christian and a Republican. He made a failed attempt for the GOP nomination for the U.S. Senate in California.
He also claimed to be a rabid anti-Communist, because of his experiences on the run behind the Iron Curtin.
"Red-fighting that's what I'm doing," Cleaver said in an interview during his congressional campaign. "I have taken an oath in my heart to oppose communism until the day I die."