Thirteen people were arrested at Princeton University on Monday after occupying a campus building, an incident the university president called “completely unacceptable.”

The “Princeton Gaza Solidarity Encampment” began on Thursday when roughly 50 people set up tents in McCosh Courtyard. After immediately receiving a warning that the tents violated school policies, the protesters disassembled the tents and transitioned to holding a sit-in with singing, chants, and drums. National Review previously reported that at least three Princeton professors held class within the demonstration. Since the university policies do not permit sleeping outdoors, the protesters have taken shifts staying awake in the courtyard during the night.

The demonstration escalated Monday afternoon, when the activist group Princeton Israeli Apartheid Divest announced on social media that students would “take over Clio Hall,” specifically Dean of the Graduate School Rodney Priestley’s office.