Organized on the Internet by the group Join the Impact, the rally was one of at least 150 protests planned nationwide after the vote on Proposition 8 in California, which defined marriage as between a man and a woman.
In Chicago, protesters gathered at Federal Plaza, carrying rainbow-colored flags and signs with messages like "Fix Marriage, Not Gays" and "Repeal Proposition 8." Organizers said they hoped to achieve "full marriage equality" in Illinois.
But more importantly, they said, they wanted to show solidarity with the California gay community and pressure that state's Supreme Court to overturn Proposition 8.
"Civil rights for our community in California still hang in the balance," Andy Thayer, co-founder of the Gay Liberation Network, told demonstrators. "We're here to tell the court to rule the right way!"
Many demonstrators directed their anger toward various faiths that supported the ban with volunteers and donations. But for the most part, their mood was jovial, singing along with the Chicago Gay Men's Chorus in a rendition of "Down by the Riverside."
Across the street from Federal Plaza, a small counter-protest formed with about a dozen supporters, including Peter LaBarbera, who said he'd like to see a measure like Proposition 8 pass in Illinois.
"The people spoke in California and we believe they'd speak the same way here," said LaBarbera, a board member of Protect Marriage Illinois.
After a few hours, demonstrators left the plaza and marched through downtown. A Chicago police spokeswoman said the rally went smoothly with no arrests despite demonstrators blocking street traffic along Michigan Avenue.
Many demonstrators echoed Troy Ratcliff, 41, of Chicago, who said he would not stand by and watch years of hard work for gay marriage rights go to waste.
"We fought for a long time for this and to see California vote that down like they did ... It's wrong. It's just wrong," he said.