Peace activists joined with war veterans and people more focused on the economy for the protest, which was expected to draw thousands of participants. Marchers arrived at Grant Park with signs denouncing NATO, including ones that read: "War(equals)Debt" and "NATO, Go Home."
They planned to walk 2½ miles to the site where President Barack Obama and other world leaders were meeting to discuss the war in Afghanistan, European missile defense and other issues.
"I'll be walking all day and guiding all day, trying to keep tempers calm," said Sue Eleuterio of Highland, Ind., a longtime activist who plans to act as a "peace guide" by mediating problems between police and protesters. "Our goal, believe it or not, is to have a family friendly protest that is peaceful."
Organizers of Sunday's rally had initially predicted tens of thousands of protesters this weekend. But that was when the G-8 summit of leading industrial nations was also scheduled to be in Chicago. Earlier this year, Obama moved the Group of 8 economic meeting to Camp David, the secluded retreat in rural Maryland.
Chicago kept the NATO summit, which focuses on international security matters but not the economy. That left activists with the challenge of persuading groups as diverse as teachers, nurses and union laborers to show up for the Chicago protests even though the summit's main focus doesn't align with their most heart-felt issues.
"I'm here to protest NATO, which I feel is the enforcement arm of the ruling 1 percent — of the capitalist 1 percent," said protester John Schraufnagel, who took a bus from Minneapolis to Chicago.
Sunday's protest followed several smaller demonstrations over the previous two days, including a march Saturday to the home of Mayor Rahm Emanuel, Obama's former chief of staff.
Later that evening, hundreds of demonstrators zigzagged through downtown, some decrying terrorism-related charges leveled against three young men earlier in the day.
Increasingly tense clashes Saturday night tested police who used bicycles to barricade off streets and horseback officers to coax them in different directions. Eighteen people were arrested.
Police Supt. Garry McCarthy said officers would be ready with quick but targeted arrests of any demonstrators who turn violent Sunday.
"If anything else happens, the plan is to go in and get the people who create the violent acts, take them out of the crowd and arrest them," warned McCarthy. "We're not going to charge the crowd wholesale — that's the bottom line."
Security has been tight throughout the city. As police gathered en masse on street corners, near parks and landmarks, the city's streets remained largely vacant and many downtown buildings closed.
"It's strange because downtown is empty," said Gabe Labovitz, an economist out for a walk near his home. "The police presence is reassuring but unnerving."
Three activists who traveled to Chicago for the summit were accused Saturday of manufacturing Molotov cocktails in a plot to attack Obama's campaign headquarters, Emanuel's home and other targets.
Defense lawyers argued that the police had trumped up the charges to frighten away peaceful protesters. They told a judge it was undercover officers who brought the firebombs to an apartment in Chicago's South Side where the men were arrested.
On Sunday, police said two other men were in custody after being accused of planning to make Molotov cocktails to be used during the NATO summit.
Sebastian Senakiewicz, 24, was charged with one felony count of terrorism/making a false threat. Mark Neiweem, 28, was charged with attempted possession of explosives. Both men were scheduled to appear in court later Sunday.
It was not immediately clear if those arrests were related to the other three.
Kris Hermes of the National Lawyers Guild said the charges seem to be part of a wider effort to scare people and diminish the size of the demonstrations.