ATLANTA (Reuters) - Former world boxing champion Vernon Forrest was shot dead in Atlanta, the victim of an apparent robbery, city authorities and local media said on Sunday.

Forrest was best known for causing an upset in 2001 against then WBC welterweight champion Shane Mosley -- regarded by many as the world's best fighter at the time -- and beating him again six months later.

Forrest, who was based in Atlanta, was a world champion in the welterweight and light middleweight divisions and also fought for the United States at the 1992 Barcelona Olympics.

"The cause of death is multiple gunshot wounds involving the torso and thigh, and the manner of death is homicide," the chief medical examiner of Georgia's Fulton county said in a statement.

Forrest was shot on Saturday as he attempted to prevent people from stealing his car at a gas station in southwest Atlanta. His girlfriend's son was in the car at the time.

The case is being investigated by the Atlanta police, according to the medical examiner and local media.

(Writing by Matthew Bigg; Editing by Greg Stutchbury)