At least two people were taken into custody during the sit-in and when protesters tried to march on the street. Two more were arrested including one man who allegedly pushed an officer. About 100 protesters were involved in the demonstration.
In and around the Loop, there are increasing signs of heightened security. Several buildings have guards positioned at doors. Click here for a complete list of parking restrictions.
At the Bank of America building on South LaSalle, company flags have been taken down. The desks near street side windows are now empty, and entrances on LaSalle and Clark have been closed.
"Yes, there will be some inconveniences. There may even be some violence. But it'll be nothing compared to the violence that NATO sets out every day against the peoples of the world," said Andy Thayer, NATO protest co-organizer.
The organizers of Sunday's noon rally, which is expected to draw thousands, say the city has granted them permission to set up a stage and sound system three blocks from the summit at McCormick Place.
"We want an immediate end to the war in Afghanistan, Iraq, and all the troops home now from NATO conflicts," said Pat Hunt, NATO protest co-organizer.
In the shadow of McCormick Place, organizers downplayed concerns that a small group of fringe protesters could incite violence as they have at other summits.
"Anyone who is loading the buses to come to Chicago is signing a personal non-violent pledge in line with our mission as nurses," said Jan Rodolfo, National Nurses United.
Along with beefed up Chicago police staffing, officers from other cities, including Milwaukee and Charlotte, will be here helping out.
"The biggest challenge I think for CPD isn't people in the streets. I think it's people who are going to be wearing badges," said Hunt.
The National Lawyers Guild said observers wearing green hats will be watching for police misconduct.
"To record their actions. And to protect the constitutional rights of the demonstrators," said Sarah Gelsomino, National Lawyers Guild
A group will also be staffing a makeshift medical center in the South Loop as well as first aid tents at the protests.
"We have protocols for chemical decontamination after people get pepper-sprayed or tear-gassed," said Scott Mechanic, Chicago Action Medical.
Protesters from out of town and even out of the country have started to arrive in Chicago.
Demonstrator June Kelly is here from Ireland.
"Just puttering around and looking around this fantastic city, I think people are kind of stunned a little bit about what's happening globally," Kelly said.