BOULDER, Colo. (AP) — The pungent smell of pot that blankets a popular quadrangle at the University of Colorado-Boulder every April 20 was replaced by the stench of fish-based fertilizer Friday as administrators tried to stamp out one of the nation's largest annual campus celebrations of marijuana.

After more than 10,000 people — students and non-students — attended last year's marijuana rally on Norlin Quadrangle, university officials this year applied the stinky fertilizer to the quad to deter pot-smokers.

Three people were arrested for trespassing when they walked onto the quad, sat down and refused to leave.

"Clearly they wanted to get arrested," campus police spokesman Ryan Huff said.

One of the three protesters, Johnathan Ducombe, told reporters the crackdown is more disruptive than the any of the previous years' rallies.

Huff estimates that the university will spend about $110,000 on law-enforcement Friday, about double the amount spent last year.

Huff said another person was ticketed for marijuana possession on campus.

University officials also closed the campus Friday to all unauthorized visitors and were offering 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 bars him from making any direct references to marijuana, other drugs or to 4/20.

The measures pit Colorado's flagship university, which has tired of its reputation as a top party school, against thousands who have assembled, flash mob-style, each year to demand marijuana's legalization or simply to have a good time.

With more than 30,000 students, 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.

"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."

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"I do not see any justification for the university shutting it down," said student organizer Daniel Ellis Schwartz, who contends the measures infringe on First Amendment rights to protest. Schwartz, a physics major, and other supporters of the 4/20 smoke out plan to move it to a nearby park off-campus. He suggests there also will be some form of off-campus protest against the measures.

"We do have to play a game of chess with the authorities," Schwartz said.

Cynthia Hardey, who works in the library on the quad, said she thinks the university was overreacting and that the event would go by largely unnoticed if not for the crackdown.

"You know, I go home, they got the pot in the air, big deal. Next day everything is forgotten. But now they're making a big thing about it, and this is going down in history. So we're having police state tactics here for what? Because a couple of people want to protest the laws, these pot laws? I don't get it," said Hardey, a library technician.

Hilliard expressed admiration Friday for the fertilizer's odiferous impact.

"We're able to enrich the grounds here, which are lovely, and then we can also create an extra deterrent for people to show up today," he said. "You know, to be very candid, the goal of all of this was to make it kind of an unpleasant experience to be here today for anyone who planned to come for 4/20."

Many students at the University of Colorado and other campuses 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 an 8-foot statue of himself in downtown Austin at 4:20 p.m. local time on Friday. In Southern California, the San Bernardino County Fairgrounds will host "The 420 Festival" with bands and disc jockeys on Saturday.

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.

Like most counterculture slang, 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). Yet the code stuck for obvious reasons: Authorities and nosy parents didn't know what it meant.

In Colorado, recent 4/20 observations have 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.

At least 3,000 people gathered Friday afternoon near the Colorado state capitol in Denver for the start of a two-day marijuana rally, police said.

Officers were ticketing people they saw smoking marijuana if they did not have a medical marijuana card, Detective Raquel Lopez said. Less than a handful of tickets were issued by mid-afternoon, she said.

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

Organizers planned to entertain the Denver revelers with musical performances and contests for joint rolling and best varieties of pot.

Others are rebelling against the gatherings.

The University of Colorado's student government supports the university's anti-4/20 actions this year. 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.

At dawn, campus police officers positioned themselves at school entrances, allowing in only those with university IDs or permission. They also posted "no trespassing" signs on utility poles. Anyone on campus without proper ID could be ticketed for trespassing, which carries a maximum $750 fine and up to six months in jail, said campus police spokesman Ryan Huff. By mid-day, no one had been ticketed, Huff said.

Anyone caught smoking on campus will be ticketed, just as they would any other day, Huff said. That includes anyone with a medical marijuana card, which requires that consumption be in private.

As ground crews applied the fertilizer early Friday, philosophy major Julian Hirschbaum said it smelled like "somebody cut up a bunch of fish" and watered it.

"It's pretty gross," he said. "I mean, it disrupts a lot of people who wouldn't otherwise be disrupted by this event."

Off campus, Boulder police could also issue tickets for people smoking pot, and the Colorado State Patrol will be watching for any motorists under the influence, Huff said.

"This is not about the war on drugs. It isn't even about marijuana per se," insisted Hilliard, the university spokesman. "Ten thousand to 12,000 (people) doing anything in the academic heart of the campus would be a problem."

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