Columbia University students were told to shelter in place Tuesday evening due to "heightened activity" on the Morningside campus.

"Shelter in place for your safety due to heightened activity on the Morningside campus," the university’s emergency management operations team said in an alert Tuesday.

Officials warned that "Non-compliance may result in disciplinary action" and that students should "avoid the area until further notice."

The City College of New York said it will hold classes remotely starting on Wednesday until further notice due to "current encampment activities" on the Manhattan campus.

Campus buildings will be closed except to essential personnel, the university said.

The announcement comes nearly a week after the university, which is part of the City University of New York (CUNY) system, said a group of demonstrators calling themselves the "CUNY Gaza Solidarity Encampment" set up a tent encampment on the campus' quad.

"The demonstrators appear to be made up of students, faculty and staff from City College and across the CUNY system and a sizable contingent that is not affiliated with any CUNY campus," City College President Vince Boudreau said in a letter to the community earlier this week, noting that school leaders had begun discussions with representatives from the group.

"We are working to minimize risks to people within the encampment and to protect all members of our campus community, while also minimizing the encampment's impact on campus operations," Boudreau said.

New York City Mayor Eric Adams urged protesters at Columbia University to "walk away" after dozens of demonstrators stormed the campus' Hamilton Hall in what he called a "violent spectacle that serves no purpose."

Adams said the peaceful demonstrations were being co-opted by "professional outside agitators" seeking to "create discord and divisiveness."

"That is why I'm urging every student, every protester to walk away from this situation now. Continue your advocacy through other means," Adams said at a press briefing Tuesday evening. "We will always protect the right to protest, but we must balance that right with the right to keep students in school and our city safe."

"We cannot wait till this situation becomes even more serious. This must end now," he added.

Chief Edward Caban reiterated that protesters who are not Columbia students are working to escalate the situation and said the New York City Police Department is ready to assist the university as needed.

"We have seen the tactics changing in a way that's endangering public safety," he said.

The NYPD showed footage of what Deputy Commissioner for Intel Rebecca Weiner said showed outside actors making barriers inside Hamilton Hall and "escalating" tactics such as scaling buildings and breaking windows. Cameras in the building have also been destroyed, Weiner said.

Individuals who occupied Hamilton Hall could face charges including burglary in the third degree, trespass and criminal mischief, police said.

A Columbia University spokesperson said Tuesday evening that dozens of protesters were in Hamilton Hall.

Some protesters at the University of North Carolina Chapel Hill briefly replaced an American flag with the Palestinian flag on the campus' main quad Tuesday afternoon amid ongoing demonstrations.

The crowd chanted "U.S.A!" and sang "The Star-Spangled Banner" as the American flag was returned to the flagpole, footage shows. The flag was later removed for safekeeping, according to ABC Chapel Hill affiliate WTVD.

The university canceled classes for the rest of the day as of 3 p.m. local time due to the demonstrations on campus.

Footage from the University of Mississippi campus captures intense confrontations between pro-Palestinian protesters and counter-protesters.

The dozens of pro-Palestinian protesters gathered on the campus to call for divestment from military operations in Gaza and for the university to condemn what they call "genocide."

Footage shows a much larger counter-protest surrounding the students, with many donning American flags.

Protesters reported being subject to racism and violent threats and having food thrown at them by counter-protesters.

In one video, a Black protester can be seen recording and speaking to counter-protesters, moving past the protective barricades around pro-Palestinian protesters.

Law enforcement officers can be seen urging her to walk back as counter-demonstrators taunt her, including on student making a monkey impersonation and others chanting "lock her up," according to the Stacey Spiehler, who took the video.

Law enforcement also urged counter-protesters to stand back.

Jacob Batte, Ole Miss’ director of news and media relations, told ABC News they "cannot comment specifically about that video," but that "statements were made at the demonstration on our campus Thursday that were offensive and inappropriate."

The university said it is looking into reports about specific actions and "any actions that violate university policy will be met with appropriate action."

In a statement following the confrontations, protesters said they were met with "blind reactionism that had little to do with the genocide we were protesting as well as our demands."

-ABC News' Chris Looft and Kiara Alfonseca