According to eyewitnesses ... Marcus Hyde was out driving in Malibu early Monday morning, cruising on the PCH, when he lost control and went over an embankment. We're told he was having seizures from the crash and was airlifted to the Ronald Reagan UCLA Medical Center.

One witness says Marcus went down 200 feet from the main road. As you can see from photos of the scene, obtained by TMZ, the car was completely destroyed in the wreck.

Marcus -- who regularly shoots for Kim, Kanye and other Kardashians -- is a big gearhead and has scheduled night drives with high-end exotic cars on Sunday nights. This time he was driving a Mercedes AMG CLS ... which he's shown off on social media.

We're told Marcus had a female passenger, who was hospitalized.

His condition is unknown at this time, but a law enforcement source tells us he suffered a critical head injury.