The theme of the party is "I Want to Rock With You." It kicked off at noon on Atlantic Avenue at 36th Street, where the rock has been parked since it arrived Tuesday night.
Check out the boulder's entire route from Riverside to LACMA.
There were games, food, painting for kids, a photo-op with the giant rock and a DJ playing rock-themed music.
"Well you don't often see a 340-ton rock come through your city," said Long Beach Mayor Bob Foster.
The 22-foot tall granite rock is in the middle of a 105-mile, 11-night journey. The trailer carrying the rock has 196 wheels, and it's traveling at a speed of about 5 mph.
The rock will be part of a new exhibit called Levitated Mass, designed by artist Michael Heizer. The piece will look like it's floating in the air.
The boulder is expected to arrive in Los Angeles Saturday morning.
