* Prosecutors charge doctor with involuntary manslaughter
* Dr. Conrad Murray faces up to four years in prison
* Lawyers predict a 'battle of medical experts' (Recasts with not guilty plea, adds details)
LOS ANGELES, Feb 8 (Reuters) - The doctor hired to care for Michael Jackson was charged on Monday with killing the pop star after a lengthy investigation that found a lethal cocktail of drugs in the singer's system when he died last year.
Dr. Conrad Murray, who lives in Las Vegas, pleaded not guilty to a charge of involuntary manslaughter in the death that officials have ruled was due mainly to an overdose of the powerful anesthetic propofol, among several drugs.
Murray was heckled by Jackson fans chanting "murderer" as he entered a Los Angeles courthouse in a crisp grey suit and red tie. Once in court, he faced angry members of Jackson's family, including mother Katherine and brother Jermaine.
The doctor remained stoic during the proceeding and when addressed by the judge, spoke softly. He was admonished not to leave the country and not to give any patient an anesthetic. He posted bail of $75,000 and was allowed to leave.
To reach a guilty verdict for involuntary manslaughter -- meaning Murray killed Jackson but he did so without malice -- jurors must believe that whatever the doctor did to Jackson went beyond an accident and was criminally negligent.
Murray, 56, faces up to four years in prison if convicted. He is still allowed to practice medicine, although prosecutors filed a motion to revoke his license.
"This has been a nightmare for him for many different reasons. One of the reasons is he lost a friend" in Jackson, Murray's attorney Ed Chernoff told reporters outside the courthouse. He said Murray was headed home to Las Vegas.
Murray, a cardiologist, was hired in May 2009 to care for Jackson as he prepared for a series of comeback concerts aimed at reviving a career sidelined by the singer's 2005 trial and acquittal on charges of molesting a 13-year-old boy.
The singer was a member of Motown singing group the Jackson 5 and was a hugely successful solo artist, whose 1982 smash hit "Thriller" is still the world's best-selling album.
'BATTLE OF MEDICAL EXPERTS'
The doctor has been the focus of a police probe for months since the Los Angeles coroner's office ruled that Jackson's June 25 death was a homicide. Coroners said Jackson's death was caused by propofol and the sedative lorazepam. Painkillers, sedatives and a stimulant also were found in his body.
Murray has admitted giving the 50-year-old singer propofol to help him sleep, and authorities found bottles of the anesthetic in his doctor's bag and on the bedside table of Jackson's home, according to court records unsealed last year.
The doctor and his lawyers have insisted he did nothing wrong. Murray has told investigators he was not the first doctor to give Jackson propofol, according to court records.
Lawyers outside the case say a trial will involve each side lining up experts to bolster their arguments.
"It's clear this case will be a battle of medical experts," said noted New York defense attorney Joseph DiBenedetto.
Legal experts said the likely reason police took seven months to investigate was because prosecutors wanted to get the facts straight, but some suggested it hinted at a weak case.
Outside court, Katherine Jackson told celebrity magazine People that Murray is "a monster," and Brian Oxman, lawyer for father Joe Jackson, called the charge "a slap on the wrist."