University of Chicago police and Chicago Police Department officers were also on the scene.
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The protest, for the most part, has been peaceful, and it comes on U of C's busy alumni weekend. Demonstrators say they intend to stay here overnight.
The protesters unfurled large banners with demands including "Free Palestine," "Abolish the University" and "Decolonize from Chicago 2 Palestine." Demonstrators climbed onto a covered patio waving Palestinian flags and at one point pitching a tent.
Pro-Palestinian protesters hold flag on UChicago IOP building
Demonstrators said in a press release they're comprise of U of C students, alumni and community members. In a statement the university said "Protesters attempted to bar the entrance, damaged University property and ignored directives from [university police] to clear the way... UCPD officers were able to enter the building and the protesters inside the building exited."
Former Senator Heidi Heitkamp was in her office in the building, setting up for a previously scheduled and unrelated live interview with ABC News Live when the group walked in during that setup. She was not harmed.
At one point a makeshift barricade of chairs was set up at the back of the building, with a line of students standing there with arms interlocked. A tent was also set up on the grounds.
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The U Chicago student paper, The Chicago Maroon, reported it began with a rally and march on campus that ended with some of the marchers entering the IOP building, locking the doors, and even spray painting security cameras.
The protest also drew counter-demonstrators. A nearby building blasted the national anthem, and with the takeover happening across the street from a Jewish student center, some turned out with Israeli flags.
"It brings a lot of fear and for no reason, and I feel like it's not going to accomplish what they think it's going to accomplish at this point," said Ryan McDowell, Hyde Park resident.
U Chicago police cleared an encampment off the quad a week and a half ago. The encampment had been there for several days at that time. Organizers said their campaign to get the school to divest from companies that benefit Israel would continue, but a faculty advisor to that group told us those organizers are not involved in this action.
"It took me by surprise when it happened. It didn't take me by surprise that it happened. A lot of people are really angry at the university," said Callie Maidhof, UChicago professor and member of Faculty for Justice in Palestine.
Protest organizers declined our request for an on-camera interview, but in a press release said they intended to set up another encampment on the quad. So far, that has not happened, and we did see a noticeable security presence there.