BOULDER, Colo. (AP) — Stinky fish fertilizer and two dozen law-enforcement officers kept pot smokers away from a grassy quad at the University of Colorado on Friday, but a few hundred protesters defied the crackdown and rallied on another field, where some lit up at 4:20 p.m.

It was a far cry from last year's April 20 pot celebration, when more than 10,000 people gathered on the university's Norlin Quadrangle for the annual ritual of enjoying a smoke and demonstrating for legalizing marijuana.

That made the university the scene of one of the largest campus celebrations of cannabis in the nation — a reputation that prompted university administrators to take extraordinary steps to stamp out this year's rally.

They banned unauthorized visitors from campus, and spread smelly fertilizer on the Norlin Quad and declared it off-limits. They even booked Haitian-born hip-hop star Wyclef Jean for a free concert timed to coincide with the traditional 4:20 p.m. pot gathering.

Still, they were only partially successful. A few dozen protesters veered off a sidewalk bordering the university on Friday afternoon and marched through campus, holding signs and chanting, "Roll it. Smoke it. Legalize it."

Others joined in as the marchers made their way through the campus, and after they halted on a grassy field near a science building, the crowd reached 300, with 400 more watching from the perimeter, campus police estimated.

They counted down the seconds to 4:20 p.m., let out a cheer at zero and then lit up, exhaling a collective cloud of smoke that rose over their heads.

A few police were on hand, some in SWAT gear, but they made no move to interfere. After about 15 minutes the crowd and the smoke dispersed.

Jonathan Grell, a sophomore majoring in international affairs, said he joined the rally because it "mocks America's arcane drug laws."

He said he didn't smoke Friday but has benefited from medical marijuana.

"I participated to be counted and to make my voice heard," he said.

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James Moore, a graduate student in physics, said he went to the rally to protest the administration's decision to close down part of the campus.

"You can't do that," Moore said.

The campus is public property and students pay to attend, he said, and the university has no right to say, "No, you can't walk on the grass."

Administrators had argued they have the right to protect faculty, staff and students from disruption, and a judge said Thursday that the university could close its grounds to unauthorized visitors Friday.

University spokesman Bronson Hilliard said the steps the campus took were a success, measured by students being able to go to class and faculty being able to teach.

"For us the real achievement was there were not 10,000 to 12,000 people in the academic heart of our campus disrupting our fundamental operations," he said.

Marijuana smokefests at 4:20 p.m. on April 20, or 4/20, have become a counterculture ritual, with celebrants gathering from San Francisco's Golden Gate Park to New York's Greenwich Village.

In Austin, Texas, country music legend Willie Nelson helped unveil a downtown statue honoring him by singing his new song "Roll Me Up and Smoke Me When I Die."

Organizers said they didn't set the unveiling for April 20 because it was the date of the marijuana ritual, but once they realized it, they scheduled the unveiling at 4:20 p.m. in light of Nelson's openness about his marijuana use and advocacy for legalization.

Thousands of people gathered in Denver near the state Capitol for the start of a two-day marijuana rally.

Officers ticketed people they saw smoking marijuana if they did not have a state-issued card allowing them to consume marijuana for medical purposes, which is legal under state law.

Less than a handful of tickets were issued by mid-afternoon, Detective Raquel Lopez said.

A fair-like atmosphere prevailed at the Denver rally, with vendors selling munchies from food carts and one person peddling glass pipes from a suitcase.

The number 420 has been associated with marijuana use for decades, though the reasons 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.

Theories abound on its origin. Some say it was once police code in Southern California to denote marijuana use, probably an urban legend. It was a title number for a 2003 California bill about medical marijuana, an irony fully intended.

Others trace it to a group of California teenagers who would meet at 4:20 p.m. to search for weed, a theory as elusive as the outdoor cannabis crop they were seeking. The code stuck because authorities and nosy parents didn't know what it meant, at least for a while.

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 whether to legalize it for recreational use for adults over 21.

At the University of Colorado, three students were arrested for trespassing when they walked onto the Norlin Quad, sat down and refused to leave, campus police said. Eleven others, including two students, were ticketed for trespassing, and one was ticketed for marijuana possession on campus, police said.

One of the three arrested told reporters the crackdown was more disruptive than any of the previous years' rallies.

Campus police spokesman Ryan Huff estimated the university would spend about $110,000 on law-enforcement Friday, about double the amount spent last year.

The yellow tape was removed from the quad and officers began withdrawing shortly after 4:20 p.m.

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.

The University of Colorado's student government supported the university's anti-4/20 actions. And other Colorado students created a Facebook campaign urging their colleagues to wear formal clothing to school on Friday to repudiate the party-school reputation.

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Associated Press writers Kristen Wyatt, P. Solomon Banda and Thomas Peipert contributed to this report.