It was a test of wills and stamina as the protests raged on the eve of an even bigger march that is planned for Sunday, the opening day of the NATO summit.
Police made multiple arrests. There was one skirmish that led to one officer being led away in an ambulance.
Protesters tried to out run police on foot, bicycle and horseback.
"If they would just let people do what we came here to do and let us do what our constitution allows us to do, everything would be fine," protester Michael Acsun said.
Protesters lay on the ground, attempting block officers on bicycles. Marchers grabbed a police barricade, raising it in the air in triumph before four officers wrestled it away.
Saturday's march began around 3:30 p.m. and zigzagged throughout the downtown for more than six hours.
"We need to keep together," Zoe Sigman told the crowd as it gathered at Daley Plaza. "We need to have a moment of solidarity. This needs to be peaceful. They need our support. I would like people to think about why they're out here and how important it is to be here protesting NATO and protesting G-8."
Chicago police Supt. Garry McCarthy said there were at least 18 protest-related arrests for the weekend, with about six on Saturday. It was not clear whether some of those arrests were of people being removed from the situation and released.
McCarthy said one group swarmed a police vehicle and slashed its tire.
"We have been trying to facilitate these marches," McCarthy said. "They do not know where they're going, most of the time. The issue is, how do we deploy our resources so we can facilitate those marches? We cannot put them in front. We have to put them all over. That is part of the plan."
McCarthy also said some officers were in protective gear because some protesters were throwing items like batteries and bottles at the officers.
ABC7 could not get an estimate of how many people were injured over the course of the day. The Chicago Fire Department took one man away on a stretcher. Some witnesses said he was struck by a Chicago police vehicle, while others explained the man jumped in front of the vehicle.