Hundreds of police officers in riot gear arrested dozens of pro-Palestinian demonstrators at Columbia University on Tuesday night and cleared a building that protesters had seized about 20 hours earlier.

The occupation early Tuesday morning of Hamilton Hall, a building on Columbia’s upper Manhattan campus, escalated a crisis that has consumed the university and ignited student activism on dozens of campuses nationwide.

On Tuesday night, officers broke a second-floor window to enter the building and led demonstrators, with their hands bound by zip ties, onto law enforcement buses. The university said it had called police to campus for the second time in less than two weeks after the building was “vandalized and blockaded.”

Columbia’s president, Minouche Shafik, said in a letter to the New York Police Department that the demonstrators’ actions “have become a magnet for protesters outside our gates, which creates significant risk to our campus.” She asked the police to maintain a presence on campus through at least May 17 to prevent further encampments or occupations.

What to know about the unrest at Columbia:

— Nearly two weeks ago, the police arrested more than 100 protesters who had set up tents on the campus.

— The arrests on April 18 outraged many faculty members and students, who almost immediately pitched new tents.

— The university closed the campus Tuesday to everyone but students who live there and employees who provide essential services. It said that it would move to expel any students who had occupied Hamilton Hall, a building with a history of student takeovers.

What’s happening elsewhere:

— Clashes over the war in the Gaza Strip continued to escalate Tuesday. Police officers pepper sprayed protesters to prevent the takeover of a building at the City College of New York.

— In Oregon, demonstrators who took over a library at Portland State University overnight used wood pallets and other supplies to erect fortifications around the building’s entrance. University officials on Tuesday urged them to leave the library, which was covered in pro-Palestinian messages, and requested help from the police.

— Police officers moved into an encampment at UNC, Chapel Hill early Tuesday and arrested about 30 people, school officials said. Protesters returned later in the day, mowing down a barrier to rejoin the encampment and replacing an American flag at the center of campus with a Palestinian one.

— There were signs that campus unrest might be waning elsewhere. The police managed to end the eight-day occupation of an administration building at California State Polytechnic University, Humboldt, and in Rhode Island Brown students dismantled their encampment after administrators agreed to consider their demands.

— More than 1,000 protesters have been taken into custody on U.S. campuses since the original roundup at Columbia on April 18, according to a tally by The New York Times.

The New York Times News Service