BOULDER, Colo. -- University of Colorado police have posted 150 photos of people attending an annual gathering last week where hundreds smoked marijuana.

Police are offering $50 rewards for information about the people, many of whom are shown with hand-rolled cigarettes, glass pipes or bongs. One young man was photographed while he was streaking.

Authorities said the protesters they were trespassing on Farrand Field, which is surrounded by several dorms, despite signs and security posted to keep them away.

About 2,500 people attended the festive event, but police estimated less than half were using the drug. A local restaurant gave away sandwiches, while people dressed in costumes and danced to the beat of drums.

The gathering occurs on campus at 4:20 p.m. on April 20 each year.

Members of The Grateful Dead are believed to have organized the first 4/20 celebration in 1979 in San Jose, Calif., where "420" was rumored to be police code for marijuana smoking. A check of police codes in California, however, revealed no jurisdiction uses 420 as a code for marijuana. Some pot smokers use "420" as a code to speak about marijuana openly among unsuspecting teachers and parents, according to a popular Web site.

Police asked the public to look at the photos and call (303) 492-8168 if they recognize anyone in the pictures.