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

Columbia University has threatened student protesters with "clear consequences" after protesters occupied a building in an escalating standoff. Students defied a 2 p.m. deadline to disperse Monday.

Students occupying the building face expulsion, the university warned.

"We made it very clear yesterday that the work of the University cannot be endlessly interrupted by protesters who violate the rules. Continuing to do so will be met with clear consequences. Protesters have chosen to escalate to an untenable situation—vandalizing property, breaking doors and windows, and blockading entrances—and we are following through with the consequences we outlined yesterday," Columbia said in a statement.

Students who did not commit to the terms Columbia's proposed agreement to vacate the encampment by 2 p.m. Monday have been suspended.

"Those students will be restricted from all academic and recreational spaces and may only access their individual residence. Seniors will be ineligible to graduate," Columbia said.

"This is about responding to the actions of the protesters, not their cause," Columbia said.