Minutes after the clock struck midnight, the Clerk and Records department at Denver's municipal office building began the process of issuing licenses, The Denver Post reported.
With paperwork in hand, same-sex couples lost no time in cementing their unions, using the building's atrium to exchange vows.
Denver Mayor Michael Hancock performed many of the first ceremonies, along with Democratic U.S. Rep. Diana DeGette, municipal judges and religious officiants.
Cameras clicked, lights flashed and TV cameramen jockeyed for space as the couples made their way from the clerk's office to the atrium.
It looked like a scene from Oscar night as well-wishers crowded around the couples, the Post reported.
Fran and Anna Simon were the first in Denver to get a license and the first to have a civil union ceremony.
"I didn't know what to expect," said Fran, who was accompanied by her 5-year-old son. "I just expected being with Anna.