"Our officers approached them and said this area is closed can you please leave. It was clear they were going to be defiant of the orders so they were arrested," Ryan Huff, spokesperson for the CU Police Department, said.

Police officers were at the entrances to campus checking every ID to make sure visitors didn't enter the campus. But two hours after the three students were arrested at Norlin Quad, around 300 protesters carrying signs and chanting walked onto campus. Some people were calling for justice after the university announced last week that the campus would be closed to visitors as a way of cracking down on the annual 4/20 crowd.

"The school crackdown is a violation of our constitutional right," Laura Schneider, a first-year law student said, "It goes against our freedom to assemble."

Nearly 400 people looked on as the protesters occupied Duane Field where some people smoked marijuana. But the size of the crowd was a fraction of what has been seen in past years. More than 10,000 people, students and non-students, attended last year's marijuana rally on the CU campus.

"When you don't have thousands and thousands of people in one area you're going to have fewer problems," Huff said.

In addition to the three arrests, 10 people were ticketed for trespassing and one was ticketed for marijuana use.

On Thursday, a judge ruled CU could keep its campus closed after six residents filed a lawsuit claiming the move violated their First Amendment rights. It was an unprecedented decision made by the university, but they say the closure was a success. This weekend, CU Chancellor Phil DeStefano says he and other campus leaders will decide what their 4/20 plan will be for next year.

CU also scheduled a free Wyclef Jean concert for students. The university paid him $80,000 for the concert at the Coors Event Center. He was not allowed to mention marijuana or make any 4/20-related remarks during the show.

9NEWS' partners at the Denver Post report that around 6:30 a.m. on Friday, campus workers also applied a fish-based fertilizer to Norlin Quad - where most of the 4/20 celebrations have taken place in years prior.

CU says the fertilizer enriches the soil. It's made up of ground up ocean fish that fed on nutrient rich plankton which makes it have an extra special odor.

The university says they did it to make it an undesirable place for people to gather for 4/20.

CU says they do apply the fertilizer around this time every year, but they timed it this year to coincide with 4/20.

According to CU, the smell can last for about 36 hours.

There was also a rally at the State Capitol in Denver.