Cook County Clerk David Orr says his office issued 203 civil union licenses.
The state's first civil union ceremony was scheduled to be performed at midnight. By law, ceremonies cannot take place until June 2.
Many Illinois couples have been waiting a long time for this opportunity.
"We have been together 15 years and never thought this would be possible," said Joe Serio.
"We're here because we love each other very much, and we want to make that sort of commitment to each other," said Lisa Berube.
While the new law offers many rights and protections, Illinois civil unions will not be recognized by the federal government. And if opponents have their way, the law will be overturned in Illinois.
"The message is that the civil unions law needs to be repealed to preserve religious liberty in Illinois, and we also hope to put a measure on the ballot in 2012 which would affirm marriage as it really is between one man and one woman," said Peter LaBarbera, Americans For Truth About Homosexuality.
Meanwhile, six couples who got their licenses Wednesday morning won a contest for a free wedding ceremony - complete with photographers and a reception - at Catalyst Ranch in the West Loop. The couples planned to seal their unions just after midnight.
Hours before the big moment, Amanda Adlesick got her makeup and hair done. She and partner Katrina Miller considered it their wedding night.
"People are like 'you're getting civil unioned,' and I'm like, 'no, I'm getting married.' That's just sort of how I feel about it," said Adlesick.
Adlesick and Miller have been together three years and thought they would have to go to Iowa to seal their union. But the civil unions legislation in Illinois allowed them to stay home.
"It's not complete marriage, but it's taken the steps and for what we have now, it's I feel a good start," said Miller.
The event at Catalyst Ranch was set to look very much like a wedding, complete with programs, flowers and wedding cake. All the items were donated by vendors anxious to help celebrate the new law. Anita Vaughan was scheduled to perform the ceremony.
"In their hearts they are already husbands and wives, they are already together. This just makes it formal," said Vaughn.
On Thursday, a ceremony will take place in Millennium Park. Mayor Rahm Emanuel and Gov. Pat Quinn are among those expected to be on hand.
The Associated Press contributed to this report.
