NYPD officers entered the campus of the City College of New York late Tuesday night.

"As requested by the university, we are currently on campus to assist the university in dispersing those trespassing," NYPD Deputy Commissioner of Operations, Kaz Daughtry, said in a post on X, sharing video of the police moving in.

In a note to the campus community, university president Vince Boudreau, said protests on campus presented "heightened challenges."

"Most importantly, this is not primarily a CCNY demonstration, and perhaps not primarily a CUNY demonstration. The significant inclusion of un-affiliated external individuals means that we don’t have established connections to them," Boudreau's letter read.

He continued: "Specifically, this demonstration has been more contentious and violent than anything we’ve seen on campus before. Today, we distributed a letter to members of the encampment detailing specific examples of threats to the safety of people within and outside the encampment, so that all of them understand the full scope of the activity. We also want all of them, and those of you reading this note, to understand that in no way does our response to this particular and extraordinary threat overwrite our more fundamental commitments to free speech, academic freedom, or the right to peacefully protest that comply with CCNY and CUNY regulations."

Boudreau said the encamped demonstrators were told to take down their encampment and follow CUNY guidelines for "future activities."

The university president also said classes would be remote on Wednesday due to the situation on campus.

-ABC News' Jolie Lash and Victoria Arancio

In a letter authorizing the NYPD to enter the Columbia University campus and break up the protest, the school detailed the backstory on its recent talks with those in the encampments and the takeover Tuesday night of Hamilton Hall.

According to the letter, from Columbia President Minouche Shafik, an individual "hid in the building" at Hamilton Hall until it was closed and let others in. The group's purpose was "occupying the building," the letter read.

Two security guards were inside the building at the time, and the university said in the letter to the NYPD they were able to "secure their release."

"We believe that while the group who broke into the building includes students, it is led by individuals who are not affiliated with the University," the letter read. "The individuals who have occupied Hamilton Hall have vandalized University property and are trespassing."
Addressing the encampment on the school's Morningside campus, the university told police discussions had been carried out with group representatives, but on Sunday, they "reached an impasse."

At that time, the university says it told the group they did not have permission to "occupy spaces on campus" and were "in violation" of school rules and policies.
On Monday morning, the protesters were told they would be suspended if they didn't disperse by Monday at 2 p.m. ET.

The university said in its Tuesday night NYPD letter that those in the encampment were "suspended" and "not authorized to be on University property and are trespassing."

The university said it believed the actions of those in the protest were escalating. "These activities have become a magnet for protesters outside our gates which creates significant risk to our campus and disrupts the ability of the University to continue normal operations," the letter read.

Columbia's president concluded the letter by asking the NYPD to "help to clear all individuals from Hamilton Hall and all campus encampments."

She further asked police to have a campus presence through May 17 -- graduation is May 15 -- "to maintain order and ensure encampments are not reestablished."

Police moved onto the Columbia University campus a little before 9 p.m. ET and began making arrests.

The NYPD also used a vehicle with an extendable roof ramp to help officers breach Hamilton Hall from an upper floor. From there, the police cleared the building.

The campus encampment and hall were cleared before 11:30 p.m.

-ABC News' Jolie Lash, Aaron Katersky and Joshua Hoyos

The NYPD said Tuesday night that Hamilton Hall and the encampment have been cleared.

In a short media briefing, the NYPD said the only thing that remains are the tents at the encampment, which the university will clear.

No injuries have been reported, the NYPD said.

An official also said no tear gas was deployed Tuesday night.

-ABC News' Joshua Hoyos

There have been about four dozen arrests so far by the NYPD at Columbia University, according to police sources.

The first bus of protesters apprehended by police just left the campus area.

-ABC News' Aaron Katersky

Adelmi Ruiz, a reporter for Redding, California, ABC affiliate KRCR, was detained at Cal Poly Humboldt while filming police approaching an encampment of pro-Palestinian protesters.

A livestream showed Ruiz filming police in riot gear approaching protesters, who could be heard chanting, when an officer asks her to come over and tells her they need her out of the way.

An officer then tells Ruiz to put her phone away and put her hands behind her back because he is going to put her in flex cuffs -- plastic handcuffs used by police for protesters. Ruiz identifies herself as a reporter multiple times but is still arrested.

"You had an opportunity to leave. You were told multiple times to leave otherwise you were gonna be arrested," the officer says, according to a livestream which continued after Ruiz placed her phone in her pocket.

She responded that she was on assignment covering the protest.

"Find a different job if this causes you to break the law," the officer says.

As she is escorted away, Ruiz can be heard asking for help.

The Humboldt County sheriff has confirmed Ruiz will not face any charges.