Officers began arresting protesters on the University of California Los Angeles campus early Thursday, detaining some with their hands zip-tied behind their backs.

Those who were detained appeared to have been moving willingly with officers, ABC News station KABC reported.

As law enforcement moved into the encampment, some of the protesters began chanting, repeating the phrase "Shame on you" in chorus.

-ABC News' Jenna Harrison and Kevin Shalvey

UCLA officials issued an evacuation order for all protesters in the area of Royce Quad, saying the encampment was "unlawful."

Smoke billowed around law enforcement officials as they began breaking through the barricades on the campus encampment. It was not immediately clear whether the smoke had been dispersed by the police or the protesters.

The encampment in Dickson Plaza, along with all unauthorized tents and structures, had been declared "unlawful," campus public safety officials said in an alert.

"The University requires that everyone must leave the encampment and adjacent areas, as well as all unauthorized structures and tents immediately, until further notice," the alert said.

-ABC News’ Amanda Morris and Kevin Shalvey

California Highway Patrol officers in riot gear streamed early Thursday onto the University of California Los Angeles campus.

The officers were met by a wall of protesters, who appeared unwilling to move. Officials had earlier ordered protesters to abandon their encampment.

Officers were working to tear down barricades of plywood boards that been erected along some of the edges of that encampment.

Los Angeles Police Department officers had earlier retreated from the encampment. At one point, LAPD and protesters briefly scuffled as officers made their way out of the encampment.

-ABC News' Amanda Morris and Kevin Shalvey

At least 90 people were arrested Wednesday during a protest on the campus of Dartmouth University, local police said.

The people who were arrested included both students and non-students, Hanover Police Department said in a statement released Thursday. Charges included criminal trespass and resisting arrest, police said.

Ahead of Wednesday's planned protest, campus security had warned demonstrators that tents or encampments wouldn't be allowed.

"Once tents were erected, Dartmouth Safety & Security made multiple announcements to participants that they must dispel, and they refused," Charles B. Dennis, Hanover's chief of police, said in a media release.

Hanover police and the New Hampshire State Police continued with "multiple" announcements, but some protesters refused to disperse, he said.

-ABC News' Jessica Gorman and Kevin Shalvey

Over 300 Harvard University professors have signed a letter to the university urging "constructive dialogue" with the peaceful protesters on campus.

"We are concerned that the university has yet to meet with the students to hear their concerns. Instead, the administration has issued escalating threats of punitive disciplinary action, the severity of which the university has not seen in decades," professors wrote.

"We urge the administration to meet and engage in meaningful dialogue with peacefully protesting students," the letter said.

The letter, sent to interim President Alan Garber and interim Provost John F. Manning Tuesday, came after Harvard warned protesters to empty their encampment. The professors are urging Harvard's administration to follow the lead of Brown University and Northwestern University, where encampments ended after school leadership agreed to take steps toward divestment.

"I think so many of us signed this letter because, as faculty, we have a duty of care towards our students. The harshness and scale of the proposed punishment is unprecedented and frankly alarming; these are activities that should be met with dialogue, not punishment," Teju Cole, a Gore Vidal professor of the practice of creative writing at Harvard, said in a statement.

"We are calling on the administration to be fair-minded and set these protests in the context of numerous other protests that have happened here in the past, in response to which the university did the right thing," Cole said.