Many downtown businesses have told their employees to stay home during the second and final day of the summit -- where world leaders are discussing the war in Afghanistan, European missile defense and other security issues -- because of traffic snarls and the possibility of more protests.
More than two dozen Metra rail stations along a line that carries around 14,000 riders in from the southern suburbs on most weekdays will be closed and stations and platforms patrolled by a larger contingent of law enforcement personnel and K-9 units. The Chicago Transit Authority will have to reroute 24 buses through the summit zone.
Organizers with the Occupy Chicago movement planned to gather at a West Side park at 9 a.m., and an hour later march toward the Boeing building along the Chicago River in the city's business district to protest the company's role in building airplanes for the U.S. military.
Later, immigration rights activists will gather at the same park before boarding a bus to travel to the small village of Crete, about 35 miles south of Chicago, where federal officials are considering building a nearly 800-bed detention facility for illegal immigrants slated for deportation.
Andy Thayer, one of the main anti-NATO protest organizers, said he expected many demonstrators from out of town to leave Sunday night. But he said a strong contingent of protesters still will show up for the Boeing protest Monday morning and decried how city leaders and police officials have handled the protests.
"I am disgusted, particularly, with the upper echelon of our city," Thayer said.
On Sunday, several thousand protesters marched through downtown in one of Chicago's largest demonstrations in years, airing grievances about war, climate change and a wide range of other complaints as world leaders assembled for a NATO summit.
The protests drew together a broad assortment of participants, including peace activists joining with war veterans and people more focused on economic inequality. But the diversity of opinions also sowed doubts about whether there were too many messages to be effective.
And some of the most enduring images of the event were likely to be from the end -- when a small group of demonstrators clashed with a thick line of police who tried to keep them from the lakeside convention center where President Barack Obama is hosting the gathering.
The protesters tried to move east toward McCormick Place, with some hurling sticks and bottles at police. Officers responded by swinging their batons. After nearly two hours, the two sides were still locked in a standoff, with police blocking the protesters' path and the crowd refusing to leave. Some protesters appeared to have blood streaming down their faces.
Following the main skirmish, Police Superintendent Garry McCarthy said 45 protesters were arrested. Police said four officers were hurt, including one stabbed in the leg. Those numbers seemed certain to rise as new clashes erupted later.
Hundreds of protesters gathered late Sunday night near the Art Institute of Chicago as first lady Michelle Obama hosted a dinner for spouses of NATO leaders inside. At least 100 Chicago police officers in riot gear were also at the scene.
The protests lacked the size and single message that shaped the last major protest moment in Chicago, when nearly half a million people filled the city's downtown in 2006 to protest making it a felony to be an illegal immigrant.
For more coverage of the NATO summit in Chicago, visit ABC7Chicago.com.