Cheers erupted as Democratic Gov. John Hickenlooper signed the bill during a ceremony at the History Colorado Center near the state Capitol. Hundreds looked on, with many chanting "Equal! Equal!"
Some wiped away tears and others hugged during the signing ceremony.
"There is no excuse that people shouldn't have all the same rights," Hickenlooper told the crowd, which included dozens of gay couples and others watching from floors above.
The law takes effect May 1.
"It means I can change my name finally," said 21-year-old Amber Fuentes of Lakewood, who plans to have a civil union with Yolanda Martinez, 34.
"It's not marriage, but it still gives us a lot of the rights," Martinez said.
Colorado will join eight states that have civil unions or similar laws. Nine states and the District of Columbia allow gay marriage.
The signing in Colorado comes less than a year after the proposal was blocked in the House by Republicans.
"It's really meaningful. To have the recognition of your love and relationship just like any other relationship by the state is an important both legal and symbolic thing," said Democratic House Speaker Mark Ferrandino, a sponsor of the bill and the first gay lawmaker to hold the title of speaker in Colorado.
Supporters of civil unions say the passage in Colorado also is telling because in 1992, voters approved a ban on municipal antidiscrimination laws to protect gays. Four years later, the U.S. Supreme Court said the law, known as Amendment 2, was unconstitutional -- but not before some branded Colorado a "hate state."
Ferrandino said the shift "shows how much through hard work and through a very thoughtful approach you can change public opinion."
Civil unions grant gay couples rights similar to marriage, including enhanced inheritance and parental rights. People in civil unions also would have the ability to make medical decisions for their partners.
Most Republicans opposed the bill, saying they would've liked to see religious exemptions to provide legal protections for those opposed to civil unions. Churches are shielded under the new law, but Democrats rejected protections for businesses and adoption agencies, arguing the Republican suggestions were too broad and could provide legal cover to discriminate.
In May, Democrats said they had enough votes to pass the bill. But Republicans who controlled the House by one vote prevented debate on the measure.
Democrats took control of the House in November and retained the Senate.
Some Republicans insist the bill is too similar to marriage, and therefore violates the will of voters in 2006. Because of that constitutional amendment, civil unions are the only option for gay couples in Colorado for now. That could change with a U.S. Supreme Court ruling on gay marriage bans in the coming months.
"Even though it was specifically told to us that it wasn't about marriage, I think both sides know that it is what it is about," said Republican Rep. Lori Saine, speaking against the bill before a final vote last week.
Democratic Sen. Pat Steadman, also a gay lawmaker who sponsored the bill, said public support has grown for civil unions because same-sex couples face the same challenges as other families.
"We today are remedying an exclusion that has gone on for too long," said Steadman, who has been a leader in the gay-rights movement here since the days when voters passed the state's antidiscrimination amendment in 1992.
The bill's signing was especially poignant for Steadman, whose longtime partner, Dave Misner, died of cancer last year. They had been together for 11 years, a detail he included in the bill. The measure was Senate Bill 11.
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Copyright 2013 The Associated Press.