Outside the Canadian consulate, demonstrators covered their bodies with chocolate meant to represent oil to protest what they said is NATO's thirst for riches.
"NATO uses its agenda to pursue oil in all these other countries. That's half of why we're waging war, right?" said Justin Bianchi, protester.
Specifically, the group objects to the Keystone XL oil pipeline project, which would bring oil from Canada south to the United States. They said it would have negative the environment and the poor.
"Canada is a member of NATO and G8, and we are here today to demonstrate to the public that death and blood for oil is a dirty business," said Natalie Wahlberg, protester.
The event began on two wheels in the financial district before bicyclists, escorted by police, stopped traffic on Michigan Avenue on their way to the northeast end of the Loop.
"Obviously there was a little bit of traffic disruption but we facilitated the flow," said Supt. Garry McCarthy, Chicago Police Department. "And here we are. We're doing exactly what we said we were going to do."
The protest drew a heavy police presence, but even away from the event officers blanketed the Loop.
Near Columbus and Balbo, more than 20 snow plows parked in a row, forming what appeared to be a barrier on the north end of Butler Field, where a large protest is planned on Sunday.
More demonstrators gathered at a North Side church after arriving by bus from New York and other states.
International protesters who are in Chicago said several cities in Europe and Asia will be holding their own anti-NATO demonstrations to coincide with the Chicago protests.
"We are protesting against a military alliance which is spending every year one trillion dollars for preparing wars," said Reiner Braun, German anti-war activist.
No arrests were made.
According to protest organizers, more than a dozen busloads of demonstrators will be arriving in Chicago Thursday evening on the North Side.
Air Force F16s and other military aircraft will be flying over Chicago Friday as part of an air defense exercise related to security for the NATO summit. The exercise is scheduled to start around 9 a.m. and last two hours.