Keith Emerson, founder and keyboardist of the progressive-rock band Emerson, Lake and Palmer, has died aged 71.

Emerson's longtime partner, Mari Kawaguchi, called police to his home in Santa Monica, California, at about 1.30am local time on Friday.

Emerson had an apparent self-inflicted gunshot wound to the head, and authorities are investigating his death as a possible suicide.

Kawaguchi told police that Emerson could have died anywhere between Thursday evening and Friday morning.

Emerson, drummer Carl Palmer and vocalist/guitarist Greg Lake were giants of progressive rock in the 1970s, recording six platinum-selling albums.

They and other hit groups such as Pink Floyd, the Moody Blues and Genesis stepped away from rock's emphasis on short songs with dance beats, instead creating albums with ornate pieces full of complicated rhythms, intricate chords and time signature changes. The orchestrations drew on classical and jazz styles and sometimes wedded traditional rock instruments with full orchestras.

Emerson, Lake and Palmer's 1973 album "Brain Salad Surgery" included a nearly 30-minute composition called "Karn Evil 9" that featured a Moog synthesiser and the eerie, carnival-like lyric: "Welcome back my friends, to the show that never ends."

A musical prodigy, Emerson was born in Todmorden, Yorkshire in England.

By his late teens, he was playing in blues and jazz clubs in London. He helped form one of the first progressive rock groups, the Nice, before hooking up with Lake and Palmer in 1970 and debuting with them at the Isle of Wight Festival, shows that also featured Jimi Hendrix and the Who.

Although it filled stadiums, ELP also was ridiculed as the embodiment of the pomposity and self-indulgence that rock supposedly stood against. When the punk movement took off in the mid-'70s, the band was a special target, openly loathed by the Sex Pistols' Johnny Rotten among others.

Years later, Rotten (then calling himself John Lydon) and Emerson became friends, Lydon told News of the World in 2007.

"He's a great bloke," Lydon said. "I've told Keith in no uncertain terms that what put me off his band were those 20-minute organ solos and that film of their convoy of trucks crossing America."

ELP broke up in 1979, reunited in 1991, later disbanded again and reunited one last time for a 2010 tour.

* For support and information about suicide prevention, call Lifeline on 13 11 14 and the Suicide Call Back Service 1300 659 467)