BOULDER, Colo. – 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 another 400 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.”

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.

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

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.

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.

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

UC 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 Friday to repudiate the party-school reputation.