The organizing group's permit promised 200 participants, but police estimate the crowd reached 2,000.
Demonstrators took their demands for the legalization of same-sex marriage to the streets, borrowing the 'Yes, we can,' chant from Barack Obama's successful campaign and hoping to influence an upcoming court battle over California's Proposition 8.
"The court has not ruled yet on the disaster of last Tuesday, and we're here to convince the court to vote the right way," said Andy Thayer of the Gay Liberation Network.
Prop 8 was on California's last week, and 52 percent of voters approved the measure, which reads: "Only marriage between a man and a woman is valid or recognized in California."
"I feel it's an issue of separation of church and state. The state should support marriage and equal rights for everybody," said protestor Randall West.
West and his partner, Joe Nevares, have been together for 10 years, and they fear the ban on same-sex marriage in California could spread.
"It's about all the rights that go along with it. It's about having hospital visitation, financial rights and all the legal rights. That's what's important," Nevares said.
However, those against gay marriage were encouraged by the vote in California.
"In 30 states, people have voted for these amendments and made it clear that people support marriage between and a man and a woman, and we believe it would pass in Illinois, too," said Peter Labarbero of Project Marriage Illinois.
Still, Randall West and Joe Nevares, like many others in Chicago, hope to one day be able to see their names united on a marriage certificate.
"We love each other regardless, but marriage for us [is] about equal rights and legal rights," West said.
Similar but smaller rallies took place Saturday across Illinois in Springfield, Champaign and Carbondale, along with cities across the country.
The one in Chicago did not occur without consequence. The event organizer has been ticketed with disorderly conduct and demonstrating on the public street. He did not have a permit. The city may also try to recoup some of the cost associated with the need to pour police into the downtown area at the last minute to keep demonstrators safe.