Police officers at the University of Arizona in Tucson early Friday removed an unauthorized encampment on campus hours before the school’s scheduled graduation ceremony, school officials said.

The encampment structure, reportedly made from “wooden pallets and other debris,” was erected on campus property after 5 p.m. on Thursday against campus policy, the school said in a statement.

Officials said that they warned the protesters to remove the encampment and disperse, but the warnings were ignored. “This evening, police vehicles have been spiked, and rocks and water bottles have been thrown at officers and university staff,” school officials said. “Those who have violated the law are subject to arrest and prosecution. University officials have taken action to ensure the safety of Centennial Hall convocation attendees.”

The campus incidents page on the university’s website said that “chemical munitions” were deployed as police were dispersing the crowd, and the encampment was cleared shortly before 4 a.m. Eastern time.

The clearing of the encampment came less than 24 hours before the school’s scheduled graduation ceremony at 7:30 p.m. local time on Friday.

A spokesperson for the school had no additional information regarding the incident, including whether protesters were arrested. But one of the groups that organized the protest posted on Instagram early Friday asking people to show up outside Pima County Jail “until all our friends are free.” In another post, the group stated that two faculty members had been arrested.

This was the second time in less than a month that law enforcement had disbanded an encampment on the school’s campus. Several weeks ago, protesters set up an encampment at there. On April 30, law enforcement began clearing it.

Four protesters were arrested that night, including an undergraduate student, a graduate student and two people unaffiliated with the university, the school said.