Police in Pomona, some 30 miles east of Los Angeles, arrested 310 people at the two-day HARD Day of the Dead festival, headlined by popular artists including Deadmau5 and Skrillex. Another 30 miles east of Pomona, authorities in San Bernardino arrested about 180 people at the Escape: Psycho Circus, the Los Angeles Times reported. The newspaper said the charges were similar to those at the Pomona event, and no major incidents occurred there either.

Source: Yahoo Music
Link: Hundreds arrested at California electronic music festivals