HOLLYWOOD, July 21— ''Robocop,'' the story of a half-man, half-robot police officer battling evil in a futuristic Detroit, earned $8 million over the weekend to take first place at the box office, spoiling the much-publicized 50th anniversary re-release of ''Snow White and the Seven Dwarfs.''

The Walt Disney classic, which cost $1.5 million to make in 1937, came in second for the week, earning $7.5 million in ticket sales.

''Jaws the Revenge,'' the fourth movie starring the voracious man-eater, sold $7.1 million worth of tickets in its opening week to take third place. ''Revenge of the Nerds II'' dropped to fourth place, with $4.6 million in ticket sales, and ''Full Metal Jacket,'' Stanley Kubrick's Vietnam War film, was in fifth place at the box office with $4.5 million in ticket sales.