As Illinois' civil unions law takes effect, opponents have launched an effort to place a gay marriage ban in the state's constitution.

Illinois lawmakers approved the law that gives gay and lesbian couples most of the state rights of marriage in December. The law takes effect on Wednesday, but a one day waiting period makes Thursday, June 2 the first day gay couples can tie the knot. Chicago Mayor Rahm Emanuel and Governor Pat Quinn will attend an event celebrating the civil unions of 30 gay couples on Thursday.

At a Friday rally in downtown Chicago that got started at St. Peter's Catholic Church, opponents rallied against the law and launched a petition drive to constitutionally ban gay marriage.

“The civil unions bill has been shoved down our throat without the people having a chance to vote on it,” Richard Walsh of Americans for Life told Chicago ABC affiliate WLS-TV.

“We're announcing an advisory referendum petition to put on the ballot in 2012 to call on the legislature to pass a binding marriage amendment which would protect marriage – the definition between one man and one woman,” said Americans for Truth about Homosexuality President Peter LaBarbera.

“There's a battle between gay rights and religious freedom. We want to give Illinois voters the same opportunity other states have had. Whether they are red or blue, states seem to go the same way,” LaBarbera told the Chicago Tribune.