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.

After police and protesters clashed on the campus of the University of Texas at Austin following a dispersal order, leading to some arrests on Monday, the school issued a statement, saying it "took swift action to preserve a safe, conducive learning environment for our 53,000 students as they prepare for final exams."

"UT Austin requested backup assistance from the Texas Department of Public Safety to protect the safety of the campus community and enforce our Institutional Rules, such as the rule that prohibits encampments on campus. Because of the encampments and other violations of the University’s Institutional Rules related to protests, protestors were told repeatedly to disperse. When they refused to disperse, some arrests were made for trespassing. Others were arrested for disorderly conduct," the university's statement read.

UTA said protests since the start of the Israel-Hamas war have happened "largely without incident."

"The University strongly supports the free speech and assembly rights of our community and we want students and others on campus to know that protests on campus are fully permissible, provided that they do not violate Institutional Rules or threaten the safety of our campus community," the statement concluded.

-ABC News' Marilyn Heck