Lawyers for the demonstrators say police burst into a Bridgeport neighborhood apartment, broke down the door, roughed up several people inside and took nine protesters into custody, all without a warrant.
William Vassilakis, who leases the apartment, says numerous protesters were staying in the apartment.
"I could not be more disgusted, enraged, terrified, by and generally totally disappointed with the city of Chicago," he told ABC7.
Attorneys working pro-bono for the protesters say they spent much of the day trying to find those who were arrested, and they still have yet to learn what they may be charged with.
"What we understand is that police confiscated beer-making equipment. So if that's what they're saying, activists are using to make Molotov cocktails, that's outrageous," said Kris Hermes with the National Lawyers Guild.
However, a police source tells ABC7 that officers recovered several Molotov cocktails from inside the apartment.
No charges have been filed.
Chicago's top cop says he knows little about the complaints of the arrested protesters.
"We have an inquiry that we're checking into as we speak. So obviously we'll have more information," said Police Superintendent Garry McCarthy.
Meanwhile, more protesters from other parts of the country arrived by the busload at Broadway and Wellington late Thursday night. Sixteen buses filled with members of the Occupy Movement from both coasts and numerous points between were expected.
"I'm not expecting anything to happen. But I'm just going to sort of stand my ground in a way but peacefully," said Christoper Wells, Occupy Georgia.
Earlier in the day, protesters already in Chicago covered their bodies in chocolate designed to look like oil. They demonstrated against NATO and the Keystone XL oil pipeline project, which would bring oil from Canada to the U.S.
Dozens of uniformed Chicago police stood guard nearby, though they made no arrests. But around the corner a couple blocks away back-ups were apparently standing by. Inside 15 unmarked passenger vans they were dressed in black uniforms with riot gear.
The protesters have a number of planned events scheduled for Friday. The largest is expected to be a rally at Daley Plaza starting at 10 a.m.