The attack happened at the Yosemite Recreation Center.
It was on a dirt path near the park's swimming pool where police say four teens robbed a jogger of his MP3 player and thrashed him over the head with their skateboards.
One of the four suspects is under arrest. Police say they're not gang members but could be members of a "tagging" graffiti crew. Graffiti is painted on buildings and benches at Yosemite recreation center.
For some of the residents who use the park at night, their sense of security is in question.
If you have information about the attack, you're asked to call the Los Angeles Police Dept. at (877) LAPD-24-7.