The Saturday afternoon event comes at a time when public workers across the Midwest have faced attacks on their bargaining rights.
The Chicago rally is the culmination of a week of workers' rights rallies across Illinois.
Organizers estimate as many as 50,000 people turned out. Chicago police were not able to confirm that figure, but there were several thousand people downtown that came from across Illinois and other states.
The atmosphere was festive but the message serious.
"This is a way of life. This is about our people. This is about our children. This is about our future," said Mahlon Mitchell, the president of the Wisconsin Firefighters Union.
The name of the rally was "We Are One," and the dozen or so speakers hammered home a message of solidarity.
"This is really an attack on the middle class, and we want to bring that home to everybody. Everybody who's sitting at home, everybody who's out of a job, everybody who has lost wages and benefits needs to get involved," said Deborah Stensvaag, a United Airlines union member.
Union members say they're outraged by recent efforts in Wisconsin, Indiana and Ohio to curtail the bargaining power of public-sector unions.
"This is nothing more than a political power grab by the upper class and corporations. Let me be clear: We are not only fighting for our ability to fight as a union, but we are also fighting for our fundamental civil rights," said Betsy Kippers with the Wisconsin Education Association Council.
Organizers of Saturday's rally tried to connect their message to the struggle of Dr. Martin Luther King Jr., who was assassinated 43 years ago this week.
"He had been preaching his entire life about workers' rights and in this case human rights. This is a perfect example of it," said Alfred White, a UFCW Local 1546 member.
"We're wise to be here. And he just wanted peace amongst the world, and that's why we come together as one, to fight for the same cause," said union supporter Sanchez OdumBurgin.
Some question whether stronger unions, at the expense of budget and tax payers.
"The public and unions do not have the taxpayers best interests. The rank-and-file members have no representation. They are forced to pay dues, and they are given to the Democrats. They're forced from the system," said Christina Tobin with the National Taxpayers United.
Saturday's rally was the largest of more than a dozen demonstrations across Illinois this week.
There have been similar gatherings in other states as part of the "We Are One" campaign.