BOULDER, Colo. – Protesters demanding that marijuana be legalized marched onto the University of Colorado on Friday, testing the school’s determination to push the annual April 20 pot celebration off campus.

The protesters waved signs and a few appeared to be smoking marijuana as they entered the campus. They chanted, “Roll it, smoke it, legalize it.”

Only a few dozen marchers crossed onto the university from an adjacent street, but others joined in as they made their way through the campus. By the time the protesters halted near a science building, the crowd appeared to be in the hundreds.

A handful of Boulder city police officers, some in SWAT gear, watched from a corner of the field but made no immediate move on the crowd.

The protesters avoided the Norlin Quadrangle, where last year’s rally attracted more than 10,000 people.

The university spread stinky fish fertilizer on the Norlin Quadrangle early Friday and declared it closed, surrounding it with yellow tape and stationing about two dozen officers around the perimeter. That effort appeared largely successful.

Three people were arrested for trespassing when they walked onto the quad, sat down and refused to leave. One of the three protesters, Johnathan Ducombe, said the crackdown is more disruptive than any of the previous year’s rallies.

University spokesman Ryan Huff estimated that the university would spend about $110,000 on law enforcement Friday, about double what was spent last year.

At least one other person was ticketed for marijuana possession on campus.

The yellow tape was removed from the quad and officers withdrew shortly after 4:20 p.m., the traditional time for marijuana advocates to light up.

In past years, the April 20 rally at the University of Colorado, which has more than 30,000 students, was one of the largest on any campus in the nation.

Administrators were determined to fend off this year’s event and dispel an image in some people’s minds that the school was a pot-happy party palace.

Playboy magazine named Colorado the nation’s top party school in 2011. The campus also repeatedly ranks among the top schools for marijuana use, according to a “Reefer Madness” list conducted by The Princeton Review.

In addition to closing the Norlin Quadrangle and baiting it with the smelly fertilizer, university officials closed the campus to all unauthorized visitorsn Friday and offered a free campus concert by Haitian-born hip-hop star Wyclef Jean timed to coincide with the traditional 4:20 p.m. pot gathering. His contract barred him from making any direct references to marijuana, other drugs or to 4/20.

“We don’t consider this a protest. We consider this people smoking pot in the sunshine,” said university spokesman Bronson Hilliard. “This is a gathering of people engaging in an illegal activity.”

Many students at the University of Colorado and schools across the country have long observed 4/20. The counterculture observation is shared by marijuana users from San Francisco’s Golden Gate Park to New York’s Greenwich Village.

In Austin, Texas, country music legend Willie Nelson, who’s open about his marijuana use, was expected to help unveil a statue of himself in downtown Austin at 4:20 p.m. local time Friday. In Southern California, the San Bernardino County Fairgrounds will host “The 420 Festival” with bands and disc jockeys today.

The number 420 has been associated with marijuana use for decades, though its origins are murky. Its use as code for marijuana spread among California pot users in the 1960s and spread nationwide among followers of the Grateful Dead.

In Colorado, recent 4/20 observations blossomed alongside the state’s medical marijuana industry. Approved by Colorado voters in 2000, medical marijuana boomed after federal authorities signaled in 2009 they would pursue higher-level drug crimes. All marijuana is illegal under federal law, though Colorado voters this November will consider a ballot measure to legalize it for recreational use by adults over 21.